Cache: fix data race with is_problematic_timestamp
Cache: fix data race with is_problematic_timestamp

Previously `recent_problematic_timestamp` was unprotected and accessed
potentially with multiple worker threads simultaneously.

This commit protects it with atomics and also introduces a flag to
prevent multiple timestamp checks from within the same call to hit().

Unfortunately the compiler-rt function __sync_val_compare_and_swap_16 is
not yet implemented, so I will have to take a different strategy in a
follow-up commit.

andrewrk/zig,andrewrk/zig