Change ConstString to support massive multi-threaded access
Previously ConstString had a single mutex guarding the global string
pool for each access what become a bottleneck when using it with a
large number of threads.
This CL distributes the strings to 256 individual string pools based on
a simple hash function to eliminate the bottleneck and speed up the
The goal of the change is to prepare to multi-threaded symbol parsing code
to speed up the symbol parsing speed.