This change converts the leveldb database's ExistsSha() and
ExistsTxSha to use the goleveldb API. Has() only returns if
the key exists and does not need to read the entire value into
memory resulting in less disk i/o and much less GC.
This commit contains the entire btcdb repository along with several
changes needed to move all of the files into the database directory in
order to prepare it for merging. This does NOT update btcd or any of the
other packages to use the new location as that will be done separately.
- All import paths in the old btcdb test files have been changed to the
new location
- All references to btcdb as the package name have been chagned to
database
- The coveralls badge has been removed since it unfortunately doesn't
support coverage of sub-packages
This is ongoing work toward #214.
