consolidated into the Operation structure. All reply parameters
are consolidated into the new SlapReply structure. Most operations
now have identical call signatures... Changes are not #ifdef'd,
revert to -r NO_SLAP_OP_BLOCKS if necessary to back out.

lookup the threadctx if no Op * is provided. (bdb_db_cache doesn't know
its thread context when it acquires a locker ID.)

So version.c files only need to be generated once.

must destroy existing indexes and run slapindex to regenerate.

of outstanding requests on a connection. Set rate limits for request
execution:
   no connection can have more than maxthreads/2 ops executing at once.
   a connection that is write-blocked will not execute any new ops.
   queued ops must drain before any new ops can execute.
If the queue exceeds the max_pending limit, the connection is closed.
...also fixed a bug where a connection was not marked active if it never
received a Bind.

Replaced with bi_entry_get_rw.
Implemented for back-bdb, back-ldbm, back-ldap.