Recording to the same cassette from several threads

[zudov]

Currently, as soon as withCassette finishes it dumps the accumulated cassettes to it's FilePath. This of course means that if several threads are recording to cassette which should be on the same path they'll overwrite each other. This is a common scenario as tests are often run in parallel.

• Regardless of the solution, the thing should be more paranoid and error out if somebody have touched the file while it was running and it doesn't know hot to handle it.
• If several recorders are writing to the same cassette those interactions should be merged together. This can be done either when
  • we save the cassette, by noticing that somebody have written to the file after we have started, reading that file and merging with it; or when
  • we add new recordings, by sharing the cassette for multiple threads if they are binded to the same path