Similarly as with kvdb.View this commits adds a reset closure to the
kvdb.Update call in order to be able to reset external state if the
underlying db backend needs to retry the transaction.
This commit adds the full bbolt-backed client database as well as a set
of unit tests to assert that it exactly implements the same behavior as
the mock ClientDB.
This commit renames the variables dbName to towerDBName and dbVersions
to towerDBVersions, to distinguish between the upcoming clientDBName
clientDBVersions. We also move resusable portions of the database
initialization and default endianness to its own file so that it can be
shared between both tower and client databases.
