(ITS#6389) Allow setting syncrepl keepalive

[quanah@zimbra.com]

Full_Name: Quanah Gibson-Mount
Version: 2.4.19
OS: NA
URL: ftp://ftp.openldap.org/incoming/
Submission from: (NULL) (209.131.62.113)


When configuring a replica using syncrepl, it should be possible to have an
option that sets the keepalive used for the tcp connection between the replica
and master.  Support for this was added to libldap, but no corresponding support
was added to the syncrepl stanza.  I'd suggest a keyword like "keepalive" for
configuring this, based in seconds.