Extend Fill Function

[Yuan Tian <ja...@gmail.com>]

Hi all,

In 0.13 or previous version, we only support fill in two kinds of
queries(1. single point query and group by time query) which are very
limited.

You can see the latest fill docs in this
pr(https://github.com/apache/iotdb/pull/6605).

In current master, we support fill function in all kinds of queries,
and after rethinking about the fill function and referring to some
other time-seires databases like InfluxDB, we make some changes to
fill syntax and some changes are not compatible with 0.13.

1. We only support FILL(linear) FILL(previous) FILL(constant_value),
and abandon time range, datatype in fill clause.
2. In current master, we don't support FILL(previousUntilLast), but we
may support in the future.
3. FILL clause is never limit to only single point query and group by
time. It can be used with any legal query sql.
4. If the data type is not compatible with fill way, e.g. using
fill(linear) for text data type, we neither fill it nor throw
exception, just keep it as it is. That means we will try our best to
fill it as much as possible and never throw exception even if there
exist some columns we can never fill.


Best,
---------------
Yuan Tian