Replace the linked list of TCP dispatches with hash table

Reusing TCP connections with dns_dispatch_gettcp() used linear linked
list to lookup existing outgoing TCP connections that could be reused.
Replace the linked list with per-loop cds_lfht hashtable to speedup the
lookups.  We use cds_lfht because it allows non-unique node insertion
that we need to check for dispatches in different connection states.

lib/dns/include/dns/dispatch.h

@@ -77,7 +77,8 @@ struct dns_dispatchset {
 #define DNS_DISPATCHOPT_FIXEDID 0x00000001U
 
 isc_result_t
-dns_dispatchmgr_create(isc_mem_t *mctx, isc_nm_t *nm, dns_dispatchmgr_t **mgrp);
+dns_dispatchmgr_create(isc_mem_t *mctx, isc_loopmgr_t *loopmgr, isc_nm_t *nm,
+		       dns_dispatchmgr_t **mgrp);
 /*%<
  * Creates a new dispatchmgr object, and sets the available ports
  * to the default range (1024-65535).