v1.9.2.0 : Fixing an RPC Hang in `breakoutd` + Build Improvements

This release fixes an RPC hang in breakoutd, and reworks the build system so that one source
tree can serve more than one platform. The RPC hang came from file-scale operations on wallet.dat (e.g. backupwallet, encryptwallet). This bug was introduced with v1.7.3.0, and surfaced recently.

It is a recommended upgrade for anyone running breakoutd, and an optional one for everyone else. The release notes provide specific details on the symptoms, causes, and fix.

The build system now better supports building for multiple targets from the same source tree. Also, building documentation is cleaned and improved, and obsolete makefiles are removed.

Release notes with a full description of this release: doc/release-notes/release-notes-1.9.2.0.md.