Introduce leveldb/util package
Refactoring [part 1] -- table writer, filter

* Merge block writer into table package.
* Rewrite table writer.
* API changes on filter package.
Merge package hash into util
leveldb: testing: New concurrent test
Merge pull request #31 from mischief/master

file locks for plan 9
Improves comparer

Should fix issue #28.
cache: Fixes state corruption and improves overall code of LRU cache

Hopefully whould fix issue #26.