Fixes#5653 and #5394.
Previously dropping retention policies did not propogate to local TSDB
shards. Instead, the retention policiess would just be removed from the
Meta Store.
This PR adds ensures that data associated with retention policies is
removed, when the retention policy is dropped.
Also, it cleans up a couple of other methods in `tsdb`, including the
requirement to provide (redundant) shardIDs when deleting databases.
Float values are not supported in the existing engine and the tsm1
engines. This changes NewPoint to return an error if a field value
contains a NaN field. It also allows us to validate fields to prevent
other unsupported types from sneaking in through other input plugins.