remove forestdb for now

not any benfefit in maintaining this for the time being
added boltdb benchmark, same as others
major refactor of kvstore/index internals, see below

In the index/store package
introduce KVReader
  creates snapshot
  all read operations consistent from this snapshot
  must close to release

introduce KVWriter
  only one writer active
  access to all operations
  allows for consisten read-modify-write
  must close to release

introduce AssociativeMerge operation on batch
  allows efficient read-modify-write
  for associative operations
  used to consolidate updates to the term summary rows
  saves 1 set and 1 get op per shared instance of term in field

In the index package
introduced an IndexReader
  exposes a consisten snapshot of the index for searching

At top level
  All searches now operate on a consisten snapshot of the index