Add timer for Insert Cache flush

1) Add timer to flush every time period. (in seconds)
2) Add option to not flush upon select.
3) Ensure at least one of the above is on, but allow for both.

Suggested names for variables are:
columnstore_cache_flush_type=SELECT
columnstore_cache_flush_type=TIMER
columnstore_cache_flush_type=BOTH <default>
columnstore_cache_flush_time=300

Documentation: When Cache Insert is on, all traffic should go to primary PM only.

If columnstore_cache_flush_type=SELECT, MaxScale can be configured using the CCR filter:

https://mariadb.com/kb/en/mariadb-maxscale-2208-consistent-critical-read-filter/