RocksJava - FindBugs issues

Addressed some FindBugs issues.
[RocksJava] Remove obsolete dbFolder cleanup
[RocksJava] Comparator tests for CF

- Added AbstractComparatorTest.
- Fixed a bug in the JNI Part about Java comparators
[RocksJava] Quality improvements

Summary:
- Addressed some FindBugs issues.
- Remove obsolete dbFolder cleanup
- Comparator tests for CF
 - Added AbstractComparatorTest.
 - Fixed a bug in the JNI Part about Java comparators
- Minor test improvements

Test Plan:
make rocksdbjava
make jtest
mvn -f rocksjni.pom package

Reviewers: adamretter, yhchiang, ankgup87

Subscribers: dhruba

Differential Revision: https://reviews.facebook.net/D29571
Fixed a Lint problem
Integrated changes from D29571
Provide mechanism to restart tests from previous error

Summary:
While running rocksdb tests, we sometimes encounter errors and
the test run stops. We now provide a new make target call check_some
which restarts the test run from a specific test and continues from
there depending on the value of the environment variable ROCKSDBTESTS_START

Test Plan:
Run make check_some with different values of
ROCKSDBTESTS_START.

Reviewers: igor

Reviewed By: igor

Subscribers: dhruba, leveldb

Differential Revision: https://reviews.facebook.net/D29913
Add DBProperty to return number of snapshots and time for oldest snapshot

Summary:
Add a counter in SnapshotList to show number of snapshots. Also a unix timestamp in every snapshot.
Add two DB Properties to return number of snapshots and timestamp of the oldest one.

Test Plan: Add unit test checking

Reviewers: yhchiang, rven, igor

Reviewed By: igor

Subscribers: leveldb, dhruba, MarkCallaghan

Differential Revision: https://reviews.facebook.net/D29919
Add -fno-exceptions flag to ROCKSDB_LITE.

Summary: Add -fno-exceptions flag to ROCKSDB_LITE.

Test Plan: make OPT=-DROCKSDB_LITE shared_lib -j32

Reviewers: sdong, igor

Reviewed By: igor

Subscribers: dhruba, leveldb

Differential Revision: https://reviews.facebook.net/D29901
Merge pull request #422 from fyrz/RocksJava-Quality-Improvements

Rocks java quality improvements