Make reports available to CodeChecker parse incrementally during analysis

[Discookie]

Related issue for the VS Code plugin: Ericsson/CodecheckerVSCodePlugin#143

For local use of CodeChecker, when doing a full CodeChecker analyze run, sometimes a couple files take vastly longer than the rest. Until the full analysis is done, CodeChecker parse has no ability to read partial results.

In general, CodeChecker parse should support reading partial analysis results. This is not a straightforward problem, and would require reworking how metadata is stored, to become atomic.