metadata: remove thrift and add comments

[MyXOF]

close #162

[codecov-io]

Codecov Report

Merging #171 into master will increase coverage by 23.13%. The diff coverage is 59.92%.

@@             Coverage Diff             @@
##           master     #171       +/-   ##
===========================================
+ Coverage   48.33%   71.47%   +23.13%     
===========================================
  Files         259      168       -91     
  Lines       14918     5454     -9464     
  Branches     2745      708     -2037     
===========================================
- Hits         7211     3898     -3313     
+ Misses       7063     1340     -5723     
+ Partials      644      216      -428

Impacted Files	Coverage Δ
...ead/query/impl/QueryWithoutFilterExecutorImpl.java	100% <ø> (ø)
.../tsfile/timeseries/write/schema/SchemaBuilder.java	83.33% <ø> (-16.67%)	:arrow_down:
...ies/read/controller/MetadataQuerierByFileImpl.java	80.95% <ø> (ø)
...sfile/timeseries/filter/factory/FilterFactory.java	75% <ø> (ø)
...n/edu/tsinghua/tsfile/common/utils/PublicBAOS.java	50% <ø> (-33.34%)	:arrow_down:
...ua/tsfile/timeseries/write/series/ChunkBuffer.java	61.95% <ø> (ø)
.../tsfile/timeseries/read/common/MemSeriesChunk.java	83.33% <ø> (ø)
...singhua/tsfile/encoding/common/EncodingConfig.java	0% <ø> (ø)	:arrow_up:
...series/write/record/datapoint/StringDataPoint.java	50% <ø> (ø)	:arrow_up:
.../timeseries/write/series/ChunkGroupWriterImpl.java	94.44% <ø> (ø)
... and 196 more

---

Continue to review full report at Codecov.

Legend - Click here to learn more Δ = absolute <relative> (impact), ø = not affected, ? = missing data Powered by Codecov. Last update ab2ee17...23f5a23. Read the comment docs.

[MyXOF]

+1

[qiaojialin]

+1