Use thread_local EVP_MD_CTX in isc_iterated_hash()

Use thread_local EVP_MD_CTX in isc_iterated_hash()

As this code is on hot path (NSEC3) this introduces an additional
optimization of the EVP_MD API - instead of calling EVP_MD_CTX_new() on
every call to isc_iterated_hash(), we create two thread_local objects
for each thread - a basectx and mdctx, initialize basectx once and then
use EVP_MD_CTX_copy_ex() to flip the initialized state into mdctx.  This
saves us couple more valuable microseconds from the isc_iterated_hash()
call.