Enable string mempools to work correctly during initialization phase

Enable string mempools to work correctly during initialization phase

Makes string mempools work before Mem::Init() was called, as may happen
during global variable initialization or early main.cc processing. If
needed, strings allocated before the Mem::Init() call are given an extra
buffer space to make sure the allocated buffer size will not match any
string pool size during deallocation.
Shortcomings: We now waste RAM on buffer increase for early allocated
strings unless they are already bigger than the maximum supported string
pool size. Statistics for early allocations are broken. Non-string
mempools still do not support early allocations.