Ensure shared index removed on database drop
Ensure shared index removed on database drop

When using the inmem index, if one drops a database, and then creates it
again, the previous index object will be reused. This includes the
previous cardinality estimation sketches, leading to inaccurate
cardinality estimations.

li-ang/influxdb,influxdb/influxdb,vladlopes/influxdb,mark-rushakoff/influxdb,runner-mei/influxdb,archsaber/influxdb,linearb/influxdb,benbjohnson/influxdb,mark-rushakoff/influxdb,vladlopes/influxdb,mark-rushakoff/influxdb,influxdb/influxdb,li-ang/influxdb,benbjohnson/influxdb,nooproblem/influxdb,archsaber/influxdb,influxdb/influxdb,runner-mei/influxdb,benbjohnson/influxdb,mark-rushakoff/influxdb,benbjohnson/influxdb,runner-mei/influxdb,nooproblem/influxdb,vladlopes/influxdb,archsaber/influxdb,nooproblem/influxdb,influxdb/influxdb,influxdb/influxdb,chenziliang/influxdb,chenziliang/influxdb,influxdb/influxdb,linearb/influxdb,mark-rushakoff/influxdb,chenziliang/influxdb,chenziliang/influxdb,mark-rushakoff/influxdb,runner-mei/influxdb,influxdata/influxdb,nooproblem/influxdb,influxdata/influxdb,vladlopes/influxdb,linearb/influxdb,linearb/influxdb,linearb/influxdb,nooproblem/influxdb,archsaber/influxdb