Disable locking in memory_region when RUSTRT_TRACK_ALLOCATIONS=0

[brson]

The exchange heap uses a lock around every malloc and free, i.e. every vector and string acquires at least two locks during its lifetime. It looks like when we are not tracking allocations (now the default) there should be no need for a lock here.

[brson]

It looks like the only locking being done now when RUSTRT_TRACK_ALLOCATIONS is on is in realloc, which is probably never used.

[brson]

realloc is used for growing vectors

[brson]

done.