improve usability of global ovrlays for write operations; may need to anticipate SLAPI accordingly.  More work needed for add (overlays need to understand they're global and call slap_mods2entry() explicitly)

define OPERATION_BUFFER_SIZE for dummy Operations

remove syncrepl stuff from Operation struct

lots of cleanup; few improvements; fix RDN selection bug when creating connection dynamic entries; rework entries ordering - now it's more intuitive and regular; optimize counters update

SLAPI always assigning connection and operation IDs of zero for internal
operations, such operations would cause a stale contextCSN to be returned from
slap_get_commit_csn(). As a result, SLAPI internal updates would be invisible
to replicas until an external update was made. Also, SLAPI internal operations
never called slap_graduate_commit_csn() which leaked pending CSNs.

Also included in this patch is a general cleanup of some of the SLAPI code.

Note that we need to use a separate mutex on conn_nextid to avoid a deadlock
where a post-operation plugin tries to acquire connections_mutex, having locked
the per-connection mutex, while the listener thread tries to acquire the
per-connection mutex (having locked connections_mutex). connection.c needs to
be fixed to acquire mutexes in the same order.

frontend stuff moved into a database structure, essentially to allow overlays to be processed before database selection; passes all tests (ITS#3080)

misc other cleanup

clear pagedresults_state if reusing an inactive connection; prepare for more consistent behavior of pagedResults control

Also, took away abandon operations blank check against deferment.
Abandon must be deferred in some cases (such as when other operations
are pending) and should be deferred in some other cases (such as too
many threads).