Ticket #47319 - make connection buffer size adjustable
    
    https://fedorahosted.org/389/ticket/47319
    Reviewed by: mreynolds (Thanks!)
    Branch: master
    Fix Description: Added a new config attribute nsslapd-connection-buffer with
    the following values:
    0 - no buffering at all - just read a single PDU at a time
    1 - default - regular fixed size LDAP_SOCKET_IO_BUFFER_SIZE buffer
    2 - adaptable buffer - if the system recv() call returns exactly the
    amount of data we requested, assume we could have read more, and increase
    the size of the buffer up to BUFSIZ, to try to minimize the number of system
    recv calls
    2 should be better than 1 in the case where the client is sending a large
    amount of data at once - e.g. large add/modify, or cases where many
    async requests are coming in over a single connection (e.g. replication)
    Platforms tested: RHEL6 x86_64
    Flag Day: no
    Doc impact: yes - document new config parameter