[MemProf] Avoid global lock when updating MIB cache

Previously we used a global Allocator-scope mutex to lock when adding a
deallocation to the MIB cache. This resulted in a lot of contention.
Instead add and use per-set mutexes.

Along with this, we now need to remove the global miss and access count
variables and instead utilize the per-set statistics to report the
overall miss rate.

Differential Revision: https://reviews.llvm.org/D109853