crypto/secp256k1: Fix signature malleability, adopt more efficient en… (#2239)

* crypto/secp256k1: Fix signature malleability, adopt more efficient encoding

This removes signature malleability per ADR 14, and makes secp match
the encoding in ADR 15.

* (squash this) add lock

docs/architecture/adr-014-secp-malleability.md

@@ -47,7 +47,7 @@ Fork https://github.com/btcsuite/btcd, and just update the [parse sig method](ht
 
 ## Status
 
-Proposed.
+Implemented
 
 ## Consequences
 

docs/architecture/adr-015-crypto-encoding.md

@@ -67,8 +67,7 @@ This is basically Ethereum's encoding, but without the leading recovery bit.
 
 ## Status
 
-Proposed. The signature section seems to be agreed upon for the most part.
-Needs decision on Enum types.
+Implemented
 
 ## Consequences