channeldb: switch to Open/Create methods rather than New

Commit includes basic tests for Open/Create. Additionally, rather than
relying on btcwallet’s addmgr for encryption/decryption, this package
now exposes a simple crypto system interface.

channeldb/error.go

@@ -1 +1,7 @@
 package channeldb
+
+import "fmt"
+
+var (
+	ErrNoExists = fmt.Errorf("channel db has not yet been created")
+)