Fix signed-unsigned compare error
[Java] Optimize statistics collector, improve object dependency in RocksObjects

Summary:
This diff merges pull request #208.  Contributor: ankgup87

[Java] Optimize statistics collector
* Optimize statistics collector by collecting statistics of multiple DBs in a single thread rather than starting up a new thread for each DB.
* Also, fix packaging of jnilib file on OS_X platform.
* Diff review: https://reviews.facebook.net/D20265

[Java] Add documentation on interdependency of dispose call of RocksObjects
* Remove transferCppRawPointersOwnershipFrom function.
  - This function was setting opt.filter_ and thus filter_ to be null. This way there is no
    one holding reference for filter object and can thus be GC'd which is not the intention.
    Replaced it with storeOptionsInstace which stores options instance. Options class
    internally holds Filter instance. Thus when Options is GC'd, filter reference
    will be GC'd automatically.
* Added documentation explaining interdependency of Filter, Options and DB.
* Diff review: https://reviews.facebook.net/D20379

Test Plan:
described in their diff reviews

Reviewers:  haobo sdong swapnilghike zzbennett rsumbaly yhchiang

Reviewed by: yhchiang
[Java] Optimize statistics collector, improve object dependency in RocksObjects

Summary:
This diff merges pull request 208.  Contributor: ankgup87

[Java] Optimize statistics collector
* Optimize statistics collector by collecting statistics of multiple DBs in a single thread rather than starting up a new thread for each DB.
* Also, fix packaging of jnilib file on OS_X platform.
* Diff review: https://reviews.facebook.net/D20265

[Java] Add documentation on interdependency of dispose call of RocksObjects
* Remove transferCppRawPointersOwnershipFrom function.
  - This function was setting opt.filter_ and thus filter_ to be null. This way there is no
    one holding reference for filter object and can thus be GC'd which is not the intention.
    Replaced it with storeOptionsInstace which stores options instance. Options class
    internally holds Filter instance. Thus when Options is GC'd, filter reference
    will be GC'd automatically.
* Added documentation explaining interdependency of Filter, Options and DB.
* Diff review: https://reviews.facebook.net/D20379

Test Plan:
described in their diff reviews

Reviewers:  haobo sdong swapnilghike zzbennett rsumbaly yhchiang

Reviewed by: yhchiang
Add a utility function to guess optimized options based on constraints

Summary:
Add a function GetOptions(), where based on four parameters users give: read/write amplification threshold, memory budget for mem tables and target DB size, it picks up a compaction style and parameters for them. Background threads are not touched yet.

One limit of this algorithm: since compression rate and key/value size are hard to predict, it's hard to predict level 0 file size from write buffer size. Simply make 1:1 ratio here.

Sample results: https://reviews.facebook.net/P477

Test Plan: Will add some a unit test where some sample scenarios are given and see they pick the results that make sense

Reviewers: yhchiang, dhruba, haobo, igor, ljin

Reviewed By: ljin

Subscribers: leveldb

Differential Revision: https://reviews.facebook.net/D18741