Sending SIGINT to an agent run does not collect the JSON files correctly

[vjovanov]

The problem was reported here: #550 (comment)

The agent collects metadata into intermediary folders and all the metadata is then merged. Since SIGINT interrupts the agent, then all the metadata merging is completely skipped.

We need to separate the metadata merging in a different task that can be invoked from the command line:

$ gradle metadataMerge

To make this task possible we need to keep a list of folders that are currently processed with the agent. When gradle is exited with SIGINT, we need to suggest to the user to execute metadataMerge to finish collecting the metadata.

This same issue needs to be fixed for the maven.