tsdb: don't allow deletes to a database in mixed index mode
tsdb: don't allow deletes to a database in mixed index mode

TSI1 and inmem indexes have different properties during deletes.
Specifically, inmem shares a global index across all shards, where
every tsi1 index is contained to a specific shard. When deleting
a series, it may cause the last reference to the series across all
shards to be dropped, necessitating a removal from the series file.
Since the inmem index shares the index across all shards, removing
the series when it's removed from the series file is sufficient.

However, in the case of a mixed index database, if the last shard
is a TSI1 shard, the other inmem indexes are not available when we
discover that it was the last reference to the series. This ends
up leaving the series in the inmem index without a series id in
the series file, causing all sorts of misbehavior.

Rather than continue curling ourselves into a ball to try to fix
this unsupported mode, give a helpful error message to the user
that they must run their database in a non-mixed index mode to
allow deletes.

mark-rushakoff/influxdb,influxdb/influxdb,nooproblem/influxdb,mark-rushakoff/influxdb,influxdb/influxdb,nooproblem/influxdb,li-ang/influxdb,influxdb/influxdb,influxdb/influxdb,li-ang/influxdb,influxdata/influxdb,mark-rushakoff/influxdb,influxdata/influxdb,mark-rushakoff/influxdb,influxdb/influxdb,nooproblem/influxdb,mark-rushakoff/influxdb,influxdb/influxdb,mark-rushakoff/influxdb,nooproblem/influxdb,nooproblem/influxdb