Investigate Appveyor alternatives for more parallel builds: Github Actions; Travis; CircleCI

Appveyor's big downside is the single sequential job serialized across both dynamorio and drmemory repositories and across PR's and master merges. One single job is now down to ~23-24 mins thanks to #4059 (closed) which is not that different from a Travis invocation, but the lack of parallelism does cause pain.

Possible alternatives that now include Windows (all quite recently):