[C++] Fixed race condition in MemoryLimitController

[merlimat]

Motivation

There is a race condition between the reserveMemory() and the mutex condition notification done when the other thread releases the memory.

The problem is that we're doing tryReserveMemory() in a loop but without holding the mutex. It can happen that we tryReserveMemory() return false (memory is full) and before we acquire the mutex and start waiting on the condition, the other thread had already released the memory and tried to notify.

Modification

We need to do the tryReserveMemory() while holding the mutex, so that we don't have this condition. It is fine to do it without mutex the first time, so that we can avoid the mutex contention in the normal case, as long as we fall back to the mutex and check again.