Better concurrency debugging support

[harendra-kumar]

We have inspectMode to inpsect an SVar. But this is of limited use because it dumps the information only when the stream ends or when the SVar is cleaned up. It may be useful to:

• print information at regular time intervals
• print after processing a specified number of items in the stream
• if a stream is stuck we can send a signal to the process and it should dump the tree of SVars in the process wherever it is stuck.