Clarify the `git_atomic` type and functions now that we have a 64 bit
version as well (`git_atomic64`).

The number of CPUs is useful information for creating a thread pool or a
number of workers, but it's not really about threading directly.  Evict
it from the thread file

Thread-free implementation

Our git_packbuilder__cache_lock function returns a value; use git__noop.

The git__noop function is more largely useful; move it into the
util header.  (And reduce the number of underscores.)

Make the pack and mwindow implementations data-race-free

Make the odb race-free

This change fixes a packfile heap corruption that can happen when
interacting with multiple packfiles concurrently across multiple
threads. This is exacerbated by setting a lower mwindow open file limit.

This change:

* Renames most of the internal methods in pack.c to clearly indicate
  that they expect to be called with a certain lock held, making
  reasoning about the state of locks a bit easier.
* Splits the `git_pack_file` lock in two: the one in `git_pack_file`
  only protects the `index_map`. The protection to `git_mwindow_file` is
  now in that struct.
* Explicitly checks for freshness of the `git_pack_file` in
  `git_packfile_unpack_header`: this allows the mwindow implementation
  to close files whenever there is enough cache pressure, and
  `git_packfile_unpack_header` will reopen the packfile if needed.
* After a call to `p_munmap()`, the `data` and `len` fields are poisoned
  with `NULL` to make use-after-frees more evident and crash rather than
  being open to the possibility of heap corruption.
* Adds a test case to prevent this from regressing in the future.

Fixes: #5591

This change adds all the necessary locking to the odb to avoid races in
the backends.

Part of: #5592

Also add the raw hostkey to `git_cert_hostkey`

Fix the `ENABLE_WERROR=ON` build in Groovy Gorilla (gcc 10.2)

The release build has been failing with the following error:

```shell
FAILED: src/CMakeFiles/git2internal.dir/hash.c.o
/usr/bin/cc -DHAVE_QSORT_R_GNU -DSHA1DC_CUSTOM_INCLUDE_SHA1_C=\"common.h\" -DSHA1DC_CUSTOM_INCLUDE_UBC_CHECK_C=\"common.h\" -DSHA1DC_NO_STANDARD_INCLUDES=1 -D_FILE_OFFSET_BITS=64 -Isrc -I../src -I../include -I../deps/pcre -I../deps/http-parser -D_GNU_SOURCE -fsanitize=thread -fno-optimize-sibling-calls -fno-omit-frame-pointer -Werror -Wall -Wextra -fvisibility=
hidden -fPIC -Wno-documentation-deprecated-sync -Wno-missing-field-initializers -Wstrict-aliasing -Wstrict-prototypes -Wdeclaration-after-statement -Wshift-count-overflow -Wunused-const-variable -Wunused-function -Wint-conversion -Wformat -Wformat-security -Wmissing-declarations -g -D_DEBUG -O0   -std=gnu90 -MD -MT src/CMakeFiles/git2internal.dir/hash.c.o -MF s
rc/CMakeFiles/git2internal.dir/hash.c.o.d -o src/CMakeFiles/git2internal.dir/hash.c.o   -c ../src/hash.c
../src/hash.c: In function ‘git_hash_init’:
../src/hash.c:47:1: error: control reaches end of non-void function [-Werror=return-type]
   47 | }
      | ^
../src/hash.c: In function ‘git_hash_update’:
../src/hash.c:58:1: error: control reaches end of non-void function [-Werror=return-type]
   58 | }
      | ^
../src/hash.c: In function ‘git_hash_final’:
../src/hash.c:68:1: error: control reaches end of non-void function [-Werror=return-type]
   68 | }
      | ^
../src/hash.c: At top level:
cc1: note: unrecognized command-line option ‘-Wno-documentation-deprecated-sync’ may have been intended to silence earlier diagnostics
cc1: all warnings being treated as errors
[11/533] Building C object src/CMakeFiles/git2internal.dir/odb_pack.c.o
ninja: build stopped: subcommand failed.
```

The compiler _should_ be able to figure out that there is no way to reach the
end of the non-void function since `GIT_ASSERT(0)` expands to either `assert()`
or an unconditional `return -1;` (after doing constant folding and stuff,
depending on the debug level). But it's not doing so at the moment, so let's
help it.

`git_cert_x509` has the raw encoded certificate. Let's do the same for
the SSH certificate for symmetry.

This change adds support for reading multi-pack-index files from the
packfile odb backend. This also makes git_pack_file objects open their
backing failes lazily in more scenarios, since the multi-pack-index can
avoid having to open them in some cases (yay!).

This change also refreshes the documentation found in src/odb_pack.c to
match the updated code.

Part of: #5399

Introduce GIT_ASSERT macros