Move memtable related files into memtable directory

[yiwu-arbug]

Summary:
Move memtable related files into memtable directory.

Test Plan:
run existing tests.

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[ajkr]

Will we want to remove memtable->db dependencies in the future?

[yiwu-arbug]

@ajkr Good point! Will work on it.

[yiwu-arbug]

The list of memtable->db dependencies:

memtable.cc:#include "db/dbformat.h"
memtable.cc:#include "db/merge_context.h"
memtable.cc:#include "db/merge_helper.h"
memtable.cc:#include "db/pinned_iterators_manager.h"
memtable.h:#include "db/dbformat.h"
memtable.h:#include "db/range_del_aggregator.h"
memtable.h:#include "db/version_edit.h"
memtablerep_bench.cc:#include "db/dbformat.h"

memtable.h contain too many dependencies from db. Shall we move it back to db directory?
And memtablerep_bench.cc is depending on InternalKeyComparator from db/dbformat.h. Do you think moving InternalKeyComparator to public header a good idea or not? Need suggestions.

[siying]

@yiwu-arbug yes, maybe move it back. Just leave the skiplist rep.

[yiwu-arbug]

@siying cool.

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@yiwu-arbug updated the pull request - view changes - changes since last import

[yiwu-arbug]

Will land after windows build pass.

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@yiwu-arbug has imported this pull request. If you are a Facebook employee, you can view this diff on Phabricator.