This commit adds support for etcd namespaces. This is useful when
using a shared etcd database where separate users have access to
separate namespaces.
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.
