Ensure system tables can have their WALs split prior to user tables

From a comment on HBASE-14190 and repeated on HBASE-18109 from yangzhe1991:

If I am not wrong, for system tables expect meta we still share logs with user regions, so even if we have some logic to assign system tables first, we still need wait log splitting done and we can not guarantee logs of system tables are split first. So there may be a blocking task that we should make multi-log feature having priority and write entries of system table in the highest priority logs. Thanks.

This is an umbrella for working out our long term plan around how we handle WAL prioritization and recovery.