In this commit, we start validating the extra opaque data of a channel
edge policy before persisting it. We just check that the data is valid
TLV.
NOTE: we recently [started
validating](1410a0949d)
this at the lnwire level. So really, no new update will reach the DB
layer without this already being checked. But we check it again here so
that the DB API behaves correctly as its own unit.
In preparation for moving the graph related schema structs to the graph
package in an upcoming commit, we move these methods to the graph
package. The structs we will move make use of these methods but we still
import them from channeldb so as to keep the ReadElement and
WriteElement helpers working as they do today.
