threads per pool, even when ltp_max_count <= 0 ("unlimited").
Keep ltp_max_count in range [0, LDAP_MAXTHR].

(setkey/getkey expected that, but purgekey could set a NULL in the middle.)
Added some checks for input key!=NULL.

API changes, need review - I'm not sure what's indented here:
- setkey(data=NULL, kfree!=NULL) searched as if intended to reset
  the key, but updated by setting the key.  Now always updates.
- setkey(key=<not found>, data=NULL) could return either success or
  failure.  Now succeeds iff (data == NULL && kfree == NULL).

"deleted item" mark.  Add DELETED_THREAD_CTX.
Also improve the hash function a bit, and make the hash unsigned.

ldap_pvt_thread_pool_purgekey() which may only be called during pauses.

Thus, also wait for pauses to finish before accessing thread_keys in
ldap_int_thread_pool_wrapper().  This may prevent pending tasks from
being started when a pause had been requested, which seems to have been
possible.  If that was a feature, we can split ltp_pause==1 in 2 states:
in pause (causes wait), and pause requested.

Also move 'thread_keys[].id = <thread id>' from pool_submit to
pool_wrapper.  Until pool_wrapper set the ctx as well, thread context
lookup would just return NULL anyway.

so a pause will work during states FINISHING and STOPPING.

Add missing waits and signals, and move waits in pool_wrapper().
Replace if(test) with while(test) when waiting for the multi-purpose
condition variable ltp_cond.

In ldap_pvt_thread_pool_submit(), when backing out of thread creation:
  ltp_pending_count '++' -> '--'.  Signal if there are no more threads.
In ldap_int_thread_pool_wrapper():
  if() -> assert() where false would result in eternal loop.

Can set to zero to explicitly clear it. (As well as just deleting
all values.)

don't assert(0); apparently, other responses may result from a simple bind (but log them, at least; ITS#4924)