Remove CharToBool

This utility function isn't really necessary since it's easier to say `c != 0`
than it is to say `CharToBool(c)`.
Don't export utility functions

These don't have anything to do with Rocksdb and really shouldn't be a part of
the public interface.
Remove gorocksdb_get_{char,int}_at_index in favor of Go functions

Go doesn't have pointer arithmetic, but it has slices, so we can use those
instead of making expensive C calls.
Merge pull request #6 from edsrzf/utility-cleanup

Utility function cleanup
Remove global variables for tracking handlers

Previously various handlers were stored in maps with IDs, but because there was
no synchronization, it meant that some functions were not safe for concurrent
use.

Rather than add synchronization, I've removed the maps and used pointers
instead of IDs. This is concurrency-safe and cheaper since no
map lookups are required.
save the pointer of the handlers and added refs to the options