Use slave mysql servers for some of the read operations

[havetisyan]

ZMS currently only uses the master db for both read and write operations. Instead, it should provide the option to configure read-only databases that could be used for read-only operations. This presents read-after-write problem where if the replication is not fast enough after adding an entry, the subsequent read could be handled from a slave that hasn't received the update, however, the benefit of unloading the read operations to slaves is quite important.