db_bench: seekrandom can specify --seek_nexts to read specific keys after seek.

Summary:
Add a function as tittle.
Also use the same parameter to fillseekseq too.

Test Plan: Run seekrandom using the new parameter

Reviewers: ljin, MarkCallaghan

Reviewed By: MarkCallaghan

Subscribers: rven, igor, yhchiang, leveldb

Differential Revision: https://reviews.facebook.net/D25035
Ignore IntelliJ idea project files and ignore java/out folder
Feature - Implement Java API for Comparator and Slice. Allows use of
either byte[] or DirectByteBuffer for accessing underlying data.
Add locking to comparator jni callback methods which must be thread-safe
Fix to memory dealocation when creating a slice from a byte buffer
Fix code style problems identified by lint
Tests for ComparatorOptions, Comparator and DirectComparator, and by
proxy we also exercise Slice and DirectSlice
Fix code review comments raised in https://reviews.facebook.net/D22779