Feature: TSKV Optimizations to reduce the memory usage.

[zipper-meng]

Which issue does this PR close?

Uncertainly, but lower probability: #1141

Rationale for this change

1. Fix some typos.
2. Fix logs print when file created or write finished in compaction and flushing.
3. Prevent too frequently flush job executions invoked from WAL total size checking.
4. Prevent too many old compact requests stack in the compact channel (the previous compaction sometimes costs more than 1 minute).

What changes are included in this PR?

Log print

• When a new file created in flush/compaction:

  Flush/Compaction: File: {} been created (level={}).
  

• When a file write finished in flush/compaction:

  Flush/Compaction: File: {} write finished (level: {}, {} B).
  

Reduce the frequency of external flush job executions.

A force flush job will be invoked when the total file size of WALs is too large (by default the max file size is 2G), if the flush job lasted over the check interval, it starts the second flush job unnecessarily.

Now the flush job by WAL file size check will never be invoked in one minute.

Group compaction job executions.

Since the compaction job sometimes lasts more than 1 minute, the compact channel may be full stacked by old compact requests 1 minute ago.

Now the compact jobs is grouped, the new compact request will be added to the group if previous compaction group is not completed.

Are there any user-facing changes?