Hash tables for cache RBTs cannot be grown

Upon creation, each dns_rbt_t structure has its maxhashbits field initialized to the value of the RBT_HASH_MAX_BITS preprocessor macro, i.e. 32. When the dns_rbt_adjusthashsize() function is called for the first time for a given RBT (for cache RBTs, this happens when they are first created, i.e. upon named startup), it lowers the value of the maxhashbits field to the number of bits required to index the requested number of hash table slots. When a larger hash table size is subsequently requested, the value of the maxhashbits field should be increased accordingly, up to RBT_HASH_MAX_BITS. However, the loop in the rehash_bits() function currently ensures that the number of bits necessary to index the resized hash table will not be larger than rbt->maxhashbits instead of RBT_HASH_MAX_BITS, preventing the hash table from being grown once the maxhashsize field of a given dns_rbt_t structure is set to any value lower than RBT_HASH_MAX_BITS.

To reproduce, start named with max-cache-size 1G; in named.conf, take note of its memory use, then change max-cache-size to 10G and run rndc reconfig; memory use will not increase significantly, proving that the hash table was not resized according to the new value of max-cache-size.

Found while working on #2279 (closed).