Add concurrency to compacting SpatialDB

Summary: This will speed up our import times

Test Plan: Added simple unit test just to get code coverage

Reviewers: sdong, ljin, yhchiang, rven, mohaps

Reviewed By: mohaps

Subscribers: dhruba, leveldb

Differential Revision: https://reviews.facebook.net/D28869
Move NeedsCompaction() from VersionStorageInfo to CompactionPicker

Summary:
Move NeedsCompaction() from VersionStorageInfo to CompactionPicker
to allow different compaction strategy to have their own way to
determine whether doing compaction is necessary.

When compaction style is set to kCompactionStyleNone, then
NeedsCompaction() will always return false.

Test Plan:
export ROCKSDB_TESTS=Compact
./db_test

Reviewers: ljin, sdong, igor

Reviewed By: igor

Subscribers: dhruba, leveldb

Differential Revision: https://reviews.facebook.net/D28719
No CompactFiles in ROCKSDB_LITE

Summary: It adds lots of code.

Test Plan: compile for iOS, compile for mac. works.

Reviewers: rven, sdong, ljin, yhchiang

Reviewed By: yhchiang

Subscribers: dhruba, leveldb

Differential Revision: https://reviews.facebook.net/D28857
Fix broken test in 31b02d.

Summary:
CorruptionTest for backupable_db_test did not call
GarbageCollect() after deleting a corrupt backup,
which sometimes lead to test failures as the newly created backup
would reuse the same backup ID and files and fail the consistency
check.

Moved around some of the test logic to ensure that GarbageCollect()
is called at the right time.

Test Plan:
Run backupable_db_test eight times and make sure
it passes repeatedly. Also run make check to make sure other
tests don't fail.

Reviewers: igor

Reviewed By: igor

Subscribers: dhruba, leveldb

Differential Revision: https://reviews.facebook.net/D28863
Fix SIGSEGV

Summary: As a short-term fix, let's go back to previous way of calculating NeedsCompaction(). SIGSEGV happens because NeedsCompaction() can happen before super_version (and thus MutableCFOptions) is initialized.

Test Plan: make check

Reviewers: ljin, sdong, rven, yhchiang

Reviewed By: yhchiang

Subscribers: dhruba, leveldb

Differential Revision: https://reviews.facebook.net/D28875
Add a unit test for behavior when merge operator and compaction filter co-exist.

Summary: Add a unit test in db_test to verify the behavior when both of merge operator and compaction filter apply to a key when merging.

Test Plan: Run the new test

Reviewers: ljin, yhchiang, rven, igor

Reviewed By: igor

Subscribers: dhruba, leveldb

Differential Revision: https://reviews.facebook.net/D28455