Added new KEYDB.CRON command which will allow lua scripts to be scheduled to run in the future. Support is also added for ring topologies with multi-master.

Bug fixes:

• Fix failure to respect max clients configuration option (Issue #147)
• Fix compile issues on raspberry pi and older OS X machines (Issue #141 and #146)
• Fix crash propogating stale keys in active replication
• Fix issue #125
• Fix issue #137
• Fix issue #132