This commit extends the ReadTx (and ReadWriteTx) interface with
ForEachBucket which can be used to iterate through all top level
buckets. This is a missing piece from the walletdb abstraction which
will allow us to iterate all keys in a walletdb opening the possibility
to build generic tools to browse and edit walletdb files regardless of
the underlying driver.
In this commit, we add a new package-level function to emulate the
existing Batch interface for bbolt. We do this via a new super-set
interface which is then checked against in the main implementation of
the Batch method.
In this commit, we add the trio of sequence based methods that bbolt
ships with to the main bucket interface. We do this in order to easily
allow walletdb as is to be slotted into place where bbolt is currently
used, without sacrificing any functionality.
