This commit extends the etcd.BackendConfig to also provide an abort
context and integrates it with the STM retry loop in order to be able
stop LND when conflicting transactions keep the loop running.
This commit extends etcd db with namespaces without additional storage
space requirements. This is simply done by instead of using an all zero
root bucket id, we use the sha256 hash of the name space as our root
bucket id.
This commit separates all etcd related sources (sans a few stubs and
config) from the rest of the source tree and makes compilation conditional
depending on whether the kvdb_etcd build tag is specified.
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.
