Use different allocators for UDP and TCP

Each worker has a receive buffer with space for 20 DNS messages of up
to 2^16 bytes each, and the allocator function passed to uv_read_start()
or uv_udp_recv_start() will reserve a portion of it for use by sockets.
UDP can use recvmmsg() and so it needs that entire space, but TCP reads
one message at a time.

This commit introduces separate allocator functions for TCP and UDP
setting different buffer size limits, so that libuv will provide the
correct buffer sizes to each of them.

lib/isc/netmgr/netmgr-int.h

@@ -559,16 +559,6 @@ isc__nm_enqueue_ievent(isc__networker_t *worker, isc__netievent_t *event);
  * way to use an isc__networker_t from another thread.)
  */
 
-void
-isc__nm_alloc_cb(uv_handle_t *handle, size_t size, uv_buf_t *buf);
-/*%<
- * Allocator for recv operations.
- *
- * Note that as currently implemented, this doesn't actually
- * allocate anything, it just assigns the the isc__networker's UDP
- * receive buffer to a socket, and marks it as "in use".
- */
-
 void
 isc__nm_free_uvbuf(isc_nmsocket_t *sock, const uv_buf_t *buf);
 /*%<