add exception and pause injections for testing drillbit stability

Use the exception injection mechanism to add exception injections to test a variety of distributed failure scenarios.

Here are some scenarios we've worked out before:
1. Cancellation:
TC1: cancel before any result set is returned
TC2: cancel in the middle of fetching result set
TC3: cancel after all result set are produced but not all are fetched
TC4: cancel after everything is completed and fetched

As test setup, we need:

• query dataset large enough to be sent to different drillbits, e.g., TPCH 100
• queries that force multiple drillbits to work on them; e.g., count ... group by

2. Completed (in each case check all drillbits are still up and running):
TC1: success
TC2: failed query - before query is executed - while sql parsing
TC3: failed query - before query is executed - while sending fragments to other drillbits for execution
TC4: failed query - during query execution

It is currently not possible to create a scenario in which a query may hang.

To check all drillbits up and running and in a clean state, run:

select count from sys.drillbits;

select count(*) from sys.memory;