This builds on the existing work to make assertion thread safe,
by adding an extra synchronization point in the holder of
`ReusableStringStream`'s stream instances, as those are used to
build the messages, and finishing the move of message scope holders
to be thread-local.

Break specific purpose generators into random and range generator sections.

Bad handling of `TestFailureException` when translating unexpected
exceptions inside assertion macros led to the unexpected exceptions
handling erroring out through throwing the same exception again.

This was then backstopped by the machinery for handling uncaught
exceptions from assertions, which is normally used by the
`CATCH_CONFIG_FAST_COMPILE` machinery, where we assume that it can
only be invoked because the assertion macros are not configured to
catch assertions.

Closes #1292

We now provide thread safe assertions as a configuration option.

All the previous refactoring to make the assertion fast paths
smaller and faster also allows us to implement the fast paths
just with thread-local and atomic variables, without full mutexes.

However, the performance overhead of thread-safe assertions is
still significant for single threaded usage:

|  slowdown |  Debug  | Release |
|-----------|--------:|--------:|
| fast path |   1.04x |   1.43x |
| slow path |   1.16x |   1.22x |

Thus, we don't make the assertions thread-safe by default, and instead
provide a build-time configuration option that the users can set to get
thread-safe assertions.

This commit is functional, but it still needs some follow-up work:
 * We do not need full seq_cst increments for the atomic counters,
   and using weaker ones can be faster.
 * We brute-force updating the reporter-friendly totals from internal
   atomic counters by doing it everywhere. We should properly trace
   where this is needed instead.
 * Message macros (`INFO`, `UNSCOPED_INFO`, `CAPTURE`, etc) are not
   made thread safe in this commit, but they can be made thread safe
   in the future, by building on top of this work.
 * Add more tests, including with thread-sanitizer, and compiled
   examples to the repository. Right now, these changes have been
   compiled with tsan manually, but these tests are not added to CI.

Closes #2948