Add option to disable replication at startup

An option to disable replication at startup could be useful. The API currently has enablereplication, disablereplication, enablepoll, and disablepoll commands, but these capabilities are always enabled at startup.

If somebody wants these capabilities to be disabled when Solr starts, so they can be enabled later through the API, that is currently not possible.

Proposal: Add a "disableAtStartup" option to both master and slave config sections, defaulting to false. If set to true in the master section, it would be the same as if "disablereplication" had been called. If set to true in the slave section, it would be the same as if "disablepoll" had been called.

The most common usage would probably be to add the new parameter to slave sections, and then use enabepoll and/or disablepoll as desired.