lib: remove bt_query_executor_interrupt, add bt_query_executor_borrow_default_interrupter

It is currently possible to interrupt a running quer executor with
bt_query_executor_interrupt.  It is however not possible to reset the
default interrupter that this function sets and resume the query
execution.

Rather than add a new query executor function to reset the default query
executor interrupter, introduce a new function,
bt_query_executor_borrow_default_interrupter, to borrow that default
interrupter.  All the bt_interrupter API is therefore accessible with
this default interrupter.

This patch removes the bt_query_executor_interrupt function, since it's
no longer needed.

Change-Id: I877dfbf22e36233750a71220fc5ab0297f8c0c28
Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/2709
Tested-by: jenkins <jenkins@lttng.org>
Reviewed-by: Philippe Proulx <eeppeliteloop@gmail.com>