Add configuration option 'coalescedWriteQueueDepthLimit' to limit queue depth within the AssetSolrUpdater actor.

When Collins is receiving high volumes of asset updates, the AssetSolrUpdater actor attempts to coalesce writes
to Solr by:

1) Building up a queue of pending writes
2) Scheduling a 'Reindex' message to be delivered to itself in 10 milliseconds (configurable).

Under very heavy volumes, the 'Reindex' message can be delayed by the size of the actor's mailbox, which has
been observed to sometimes cause over a minute of delay in actually updating the Solr index.

During this time period, data between MySQL and Solr will be inconsistent. This patch attempts to reduce the
surface area where this may occur.