sanitizer_common: optimize memory drain

Currently we allocate MemoryMapper per size class.
MemoryMapper mmap's and munmap's internal buffer.
This results in 50 mmap/munmap calls under the global
allocator mutex. Reuse MemoryMapper and the buffer
for all size classes. This radically reduces number of
mmap/munmap calls. Smaller size classes tend to have
more objects allocated, so it's highly likely that
the buffer allocated for the first size class will
be enough for all subsequent size classes.

Reviewed By: melver

Differential Revision: https://reviews.llvm.org/D105778