s3/smbd: make write time fetching async

s3/smbd: make write time fetching async

Finally use the new async dbwrap_parse_record_send/recv() functions
respectively the fetch_share_mode_send/recv wrappers for fetching the
write time from locking.tdb.

Previously for a directory with n files we would sit idle in the
directory enumeration loop fo n * m seconds waiting for responses from
ctdb, where m is the response time in seconds for a dbwrap request via
ctbd.

This is known to kill performance and we even have a parameter
"smbd:search ask sharemode" that can be used to disable fetching the
write time from locking.tdb.

Using fetch_write_time_send() works this way: in the directory
enumeration loop that calls smbd_dirptr_lanman2_entry() to marshall the
directory entries we

1. call fetch_write_time_send() after calling smbd_dirptr_lanman2_entry
   passing a pointer to the current position in the marshall buffer.

2. If fetch_write_time_send() has set the out parameter "stop", we exit
   the enumeration loop. This is necessary because we only send dbwrap
   requests but don't consume the results. This has the potential to
   deadlock so we must stop sending requests as soon as our ctdb send
   queue is full.

3. In the fetch_write_time_done() callback, if the recv function got a
   locking.tdb record, we push the write time into the marshall buffer
   at the offet saved in the request state.

This new feature is still off by default as it doesn't
give any improvement in the non-clustered usecase.
"smbd:async search ask sharemode" can be used to activate it,
which makes only sense with "clustering = yes" (execept for testing).

Signed-off-by: Ralph Boehme <slow@samba.org>
Reviewed-by: Stefan Metzmacher <metze@samba.org>