Auto merge of #2176 - str4d:1593-libevent, r=bitcartel

Auto merge of #2176 - str4d:1593-libevent, r=bitcartel

libevent-based http server

Cherry-picked from the following upstream PRs:

- bitcoin/bitcoin#5677
- bitcoin/bitcoin#6695
- bitcoin/bitcoin#6899
- bitcoin/bitcoin#7016
- bitcoin/bitcoin#7964
- bitcoin/bitcoin#8722
- bitcoin/bitcoin#8730
- bitcoin/bitcoin#9073
- bitcoin/bitcoin#9265
- bitcoin/bitcoin#9387
- bitcoin/bitcoin#9471
- bitcoin/bitcoin#9647
- bitcoin/bitcoin#9903
- bitcoin/bitcoin#6640
- bitcoin/bitcoin#8139
- bitcoin/bitcoin#8839

Closes #1593 and #1856.