[ADDED] Build with github actions

[levb]

Moved the build system from Travis to GitHub actions.

There are 2 main jobs:

• debug runs test coverage and sanitize with gcc, in debug mode; it also runs debug-compiled tests on clang and latest/LTS Ubuntu versions.
• release compiles in release mode, and tests various server versions, and packaging options (no TLS, no streaming)

PRs run debug by default. If you want to run the release build on a PR before merging it, set the build-release on it.

• Moved test coverage to codecov
• Added testing with Ubuntu (latest, 20.04), several versions of gcc, clang, latest and dev/main NATS server
• [testing] race detection #671 checks race detection (-fsanitize=thread)
• [testing] with a memory leak #670 checks leak and access-after-free detection (-fsanitize=address)
• [testing] verbose cmake to compare compile flags #673 turns on verbose cmake to compare compile flags
  • note that cmake stopped adding -fPIC to gcc, 2/5 it's ok
• cleaned out the use of NATS_BUILD_TLS_USE_OPENSSL_1_1_API in CI since it's always on

NOTE that since I added a "catch all" for thread sanitizer warnings, many tests have been failing in that mode both on travis and github workflows, so I filed #672 and skipped them in the sanitize=thread mode.

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (main@e30991e). Click here to learn what that means.
Patch has no changes to coverable lines.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #669   +/-   ##
=======================================
  Coverage        ?   78.77%           
=======================================
  Files           ?       40           
  Lines           ?    33915           
  Branches        ?     4774           
=======================================
  Hits            ?    26717           
  Misses          ?     2836           
  Partials        ?     4362           

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[mtmk]

LGTM