This commit adds the ExtendedBackend interface which is an extension to
the walletdb.DB interface. This paves the way to using etcd.db.View and
etcd.db.Update in the global View and Update functions without much code
rewrite.
This commit reduces the compare set size the STM will submit in
transactions by adding only the bucket keys along the bucket path to a
specific lock set. This lock set then used to filter the read set,
effectively removing all read only keys from the transaction predicate
that are not bucket keys.
By tracking if a read-write tx actually changes something, we can also
"bump" the mod revision of the bucket keys.
With this trick we essentially implement a read-write lock for our
bucket structure greatly reducing transaction processing time.
This commit adds an extended STM, similar to what available in etcd's
clientv3 module. This incarnation of said STM supports additional
features, like positioning in key intervals while taking into account
deletes and writes as well. This is a preliminary work to support all
features of the kvdb interface.
In this commit, we create a new package `kvdb`, which is meant to serve
as the basis for any future database abstractions within `lnd`. Rather
than directly use the `walletdb` package (which we base off of), we
instead use a series of type-aliases to re-type the fundamental
types/interfaces of the `walletdb` package. This lets us type
`kvdb.RwTx` instead of `walletdb.ReadWriteTransaction` everywhere.
Additionally, our usage of type-aliases is also intended to create an
easy pathway in the future wherein we can gradually re-defined or
re-implement these types to wean off of the `walletdb` package.
