This commit moves all localized instances of mock implementations of
the Signer interface to the lntest/mock package. This allows us to
remove a lot of code and have it housed under a single interface in
many cases.
In this commit, we split the database storage into two classes: remote
and local data. If etcd isn't active, then everything is actually just
local though we use two pointers everywhere. If etcd is active, then
everything but the graph goes into the remote database.
