v19.1.0

Release notes:

Stability improvements

• fix misses from earlier PR: log direct to stdout for scripts
• Bump dependencies (asio 1.22.1, etc)
• Make xdr client friendly
• Discard envelopes with unsigned txsets and do not fetch them
• minor fix: properly handle output file in WriteVerifiedCheckpointHashesWork
• Clarify bucketlist duration arithmetic, again.
• Clarified the required Rust toolchain in Win docs
• Refactor TxSetFrame
• Work around bugs in automake 1.15 on ubuntu 18.04 LTS
• rust/C++ bridge
• Fixes protocol upgrade bug in BucketManager test

New features

• Add a new metric to track transaction latency

Breaking changes

• Move XDR files to separate subdirs for vCurr and vNext - as to reduce the risk of accidental deployment of future protocol upgrades