Auto merge of #2335 - str4d:2333-validation-speed, r=str4d

Switch to libsecp256k1-based validation for ECDSA

Cherry-picked from the following upstream PRs:

- bitcoin/bitcoin#6983
- bitcoin/bitcoin#6954

Part of #2333.

doc/release-notes.md

@@ -0,0 +1,20 @@
+(note: this is a temporary file, to be added-to by anybody, and moved to
+release-notes at release time)
+
+Notable changes
+===============
+
+Signature validation using libsecp256k1
+---------------------------------------
+
+ECDSA signatures inside Zcash transactions now use validation using
+[https://github.com/bitcoin/secp256k1](libsecp256k1) instead of OpenSSL.
+
+Depending on the platform, this means a significant speedup for raw signature
+validation speed. The advantage is largest on x86_64, where validation is over
+five times faster. In practice, this translates to a raw reindexing and new
+block validation times that are less than half of what it was before.
+
+Libsecp256k1 has undergone very extensive testing and validation upstream.
+
+A side effect of this change is that libconsensus no longer depends on OpenSSL.