Avoid transaction state cache refresh on co-procesor startup

Transaction state cache, on startup, refreshes the transaction snapshot from HDFS. The transaction state cache startup is called during HBase co-processor startup. The refresh of the transaction snapshot can block, thus blocking the co-processor startup.

The subsequent transaction snapshot refreshes happen in the background. Also - there is no guarantee that a snapshot will be present during the refresh on startup. Hence it is okay not to refresh the snapshot during snapshot, but do it in background.