Update uthash.h to permit preallocation

[sbergercnmc]

Placed HASH_INITIAL_NUM_BUCKETS and HASH_INITIAL_NUM_BUCKETS_LOG2 into a #ifndef block so as to permit these values to be optionally defined in other code before inclusion of uthash.h and thus permit preallocation of hashtable when a lot of keys are expected to be added.

[Quuxplusone]

Seems plausible at first glance. Obvious questions:

• Have you used this in production? Does it help?
• Surely HASH_BKT_CAPACITY_THRESH should get the same treatment?
• Needs some test coverage.

[sbergercnmc]

Doing this for my use case, where I am loading over 300,000,000 keys (uint64_t), my program's loading time goes from 178 seconds to 93 seconds... Agreed that same idea can be applied to HASH_BKT_CAPACITY_THRESH, but I haven't needed this for my use case.