We introduce a new package paymentsDB and start by moving the
payment specifc errors from the channeldb package to the
paymentsDB package. We also fix linter issues which showed up
due to changing this code part.
We move all kv particular code which was in the payments.go file
to the kv related file. Code which will be backend independant
will remain in the payments.go file although only the kv backend
is currently supported.
In the following commits we will gradually unify the current
payment db operations into an interface to later down the road
support both backends (sql+kv).
