abg-workers: Rework the worker queue to improve concurrent behaviour

abg-workers: Rework the worker queue to improve concurrent behaviour

This patch refactors the abigail::workers::queue and
abigail::workers::worker implementations to avoid holding locking
primitives longer than necessary.

In particular, the queue_cond_mutex was held during the entiry worker
runtime, effectively serializing the workers. Hence, use a mutex+cond
pair for each, the input and output queue and only synchronize around
the interaction with their corresponding queues. The
tasks_todo_(mutex|cond) are meant to synchronize scheduling and
distribution of work among workers, while tasks_done_(mutex|cond) are
used for synchronizing threads when putting back the tasks to the output
queue and to hold back threads waiting for the queue and workers to
drain.

Along that way, I did some cleanup that was now possible.
 - Move entire implementation of abigail::workers::task into header.
 - Make default_notify a static member.
 - Replace the multiple constructors with one with default arguments.

	* include/abg-workers.h (workers::task): move entire
	implementation to header and drop superfluous forward declaration.
	* src/abg-workers.cc (workers::task):: Likewise.
	(workers::queue::priv): Drop queue_cond_mutex, rename queue_cond
	to tasks_todo_cond, add task_done_cond, make default_notify
	static.
	(workers::queue::priv::priv): Add default arguments to fully
	qualified constructor, drop the remaining ones.
	(workers::queue:prive::more_tasks_to_execute): Drop method.
	(workers::queue:prive::schedule_task): Do not synchronize access
	to the queue condition variable, but only on the mutex.
	(do_bring_workers_down): Likewise. Also await tasks_done to be
	empty.
	(workers::queue:prive::worker::wait_to_execute_a_task): Await
	tasks on the tasks_todo with tasks_todo_(cond|mutex) and signal
	task completion to tasks_done_cond.

Signed-off-by: Matthias Maennich <maennich@google.com>
Signed-off-by: Dodji Seketeli <dodji@redhat.com>