Feature: optimize table t compact segment only

[dantengsky]

Summary

new option for optimizing table

 optimize table t compact [segment]

to compact table meta data only (mainly the segments). This operation is expected

• to be much lighter than optimize table t compact,
• and can be invoked concurrently while the same table is being appended.

There are the cases that a table is kept being appended will large data chunks(but not large enough to construct a compact segment -- at least 1000 blocks, for the default table options), and at a relatively high frequency(say several commits per sec); the blocks generated are mostly non-fragmented, but the segments are not compact (usually only contain 1 ~10 meta data of blocks).

the un-compact segments harm both the reading and the data ingestion of the table

• for the former, reading lots of fragmented segments harms
• for the latter, the size of snapshot metadata, and the time to write it down, increases,