ITS#5655
ITS#5887
ITS#5462
ITS#5789

Pass LDAPURLDescs to connect functions instead of host/port

Add ldap_pvt_thread_pool_pausing(): pause check for slapd without locking.

Unifdef SLAP_LIGHTWEIGHT_DISPATCHER
Use thread-specific data in pool_context()
Added native thread-specific data support
Concurrency tweaks:
  store conn->c_sd, don't use ber_sockbuf_ctrl to retrieve it.
  use per-thread free lists for operations, no mutexes needed.

this number tunable.)