Re-org the table tests

Summary:
We'll divide the table tests into 3 buckets, plain table test, block-based table test and general table feature test.
This diff does no real change and only does the rename and reorg.

Test Plan: run table_test

Reviewers: sdong, haobo, igor, dhruba

CC: leveldb

Differential Revision: https://reviews.facebook.net/D15417
Some refactorings on plain table

Summary:
Plain table has been working well and this is just a nit-picking patch,
which is generated during my coding reading. No real functional changes.
only some changes regarding:

* Improve some comments from the perspective a "new" code reader.
* Change some magic number to constant, which can help us to parameterize them
  in the future.
* Did some style, naming, C++ convention changes.
* Fix warnings from new "arc lint"

Test Plan: make check

Reviewers: sdong, haobo

CC: leveldb

Differential Revision: https://reviews.facebook.net/D15429