Summary:
* Add a class SizeUnit to store frequently used consts. Currently
it has KB, MB, GB, TB, and PB.
* Change the parameter type of Options.writeBufferSize and Options.blockSize
from int to long.
Test Plan:
make rocksdbjava
make jtest
Reviewers: haobo, ankgup87, sdong, dhruba
Reviewed By: haobo
CC: leveldb
Differential Revision: https://reviews.facebook.net/D17703
