Create an API for plugging custom I/O handler

Currently Clover uses a standard Java IO/NIO libraries to read from and write to: a coverage database, coverage recording files (global and per-test), test optimization snapshots.

This is not sufficient when user want to run an application on a custom file system, such as Apache HDFS.

Provide an API which would allow to provide a custom IO handler.

Scope:

• writing coverage files (global and per-test)
• reading/writing database (optional - we deal a lot with file channels which might be a problem; a growable coverage recorder can be used at runtime instead of this)
• writing optimization snapshot (optional - not everyone uses test optimization)

References:

• https://answers.atlassian.com/questions/255286/does-clover-support-to-read-and-write-clover-db-and-coverage-recording-files-from-hdfs
• https://answers.atlassian.com/questions/48360/how-can-i-get-functional-test-code-coverage-reports-for-mapreduce-program