Protect thread_keys with ldap_pvt_thread_pool_mutex, except in
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.