Pry console on Ctrl+C for pausing or amending config

Once an LHM migration is running, it's currently impossible to react to an overloaded DB by changing the `stride` and `throttle` options. The only way to keep load under control is stop and go using `Ctrl+Z` and `fg`.

This PR introduces a Pry-based console on Ctrl+C that exposes the Chunker object. You can then set `@stride` and `@throttle`.

Apart from the introduction of a signal handler and the Pry invocation, it was also neccessary to change how the chunks' lower and upper boundaries are being computed.

Todo: specs for signal handler and Pry.