refactor: move various duties from ChannelRouter to graph.Builder

This commit is a large refactor that moves over various responsibilities
from the ChannelRouter to the graph.Builder. These include all graph
related tasks such as:
- graph pruning
- validation of new network updates & persisting new updates
- notifying topology update clients of any changes.

This is a large commit but:
- many of the files are purely moved from `routing` to `graph`
- the business logic put in the graph Builder is copied exactly as is
  from the ChannelRouter with one exception:
- The ChannelRouter just needs to be able to call the Builder's
  `ApplyChannelUpdate` method. So this is now exported and provided to
the ChannelRouter as a config option.
- The trickiest part was just moving over the test code since quite a
  bit had to be duplicated.

graph/errors.go

@@ -0,0 +1,83 @@
+package graph
+
+import "github.com/go-errors/errors"
+
+// errorCode is used to represent the various errors that can occur within this
+// package.
+type errorCode uint8
+
+const (
+	// ErrOutdated is returned when the routing update already have
+	// been applied, or a newer update is already known.
+	ErrOutdated errorCode = iota
+
+	// ErrIgnored is returned when the update have been ignored because
+	// this update can't bring us something new, or because a node
+	// announcement was given for node not found in any channel.
+	ErrIgnored
+
+	// ErrChannelSpent is returned when we go to validate a channel, but
+	// the purported funding output has actually already been spent on
+	// chain.
+	ErrChannelSpent
+
+	// ErrNoFundingTransaction is returned when we are unable to find the
+	// funding transaction described by the short channel ID on chain.
+	ErrNoFundingTransaction
+
+	// ErrInvalidFundingOutput is returned if the channel funding output
+	// fails validation.
+	ErrInvalidFundingOutput
+
+	// ErrVBarrierShuttingDown signals that the barrier has been requested
+	// to shutdown, and that the caller should not treat the wait condition
+	// as fulfilled.
+	ErrVBarrierShuttingDown
+
+	// ErrParentValidationFailed signals that the validation of a
+	// dependent's parent failed, so the dependent must not be processed.
+	ErrParentValidationFailed
+)
+
+// graphError is a structure that represent the error inside the graph package,
+// this structure carries additional information about error code in order to
+// be able distinguish errors outside of the current package.
+type graphError struct {
+	err  *errors.Error
+	code errorCode
+}
+
+// Error represents errors as the string
+// NOTE: Part of the error interface.
+func (e *graphError) Error() string {
+	return e.err.Error()
+}
+
+// A compile time check to ensure graphError implements the error interface.
+var _ error = (*graphError)(nil)
+
+// newErrf creates a graphError by the given error formatted description and
+// its corresponding error code.
+func newErrf(code errorCode, format string, a ...interface{}) *graphError {
+	return &graphError{
+		code: code,
+		err:  errors.Errorf(format, a...),
+	}
+}
+
+// IsError is a helper function which is needed to have ability to check that
+// returned error has specific error code.
+func IsError(e interface{}, codes ...errorCode) bool {
+	err, ok := e.(*graphError)
+	if !ok {
+		return false
+	}
+
+	for _, code := range codes {
+		if err.code == code {
+			return true
+		}
+	}
+
+	return false
+}