Given once Ruby allocates some memory, it doesn't give it back, bigger
set of larger actions can lead to quite big memory consumption that
persists and can accumulate over time. With this, it's hard to keep
memory consumption fully under control, especially in an environment
with other systems (passenger, pulp, candlepin, qpid). Since the
executors can terminate nicely without affecting the tasks itselves,
it should be pretty easy to extend it to watch the memory consumption.

The idea:

1. config options:
max_memory_per_executor - the threshold for the memory size per executor
min_executors_count - minimal count executors (default 1)
minimal_executor_age - the period it will check whether the memory consumption didn't grow (default 1h)

2. the executor will periodically check it's memory usage,
(http://stackoverflow.com/a/24423978/457560 seems to be a sane
approach for us)

3. if memory usage exceeds `max_memory_per_executor`, the executor is
older than `minimal_executor_age` (to prevent situation, where the
memory would grow too fast over the max_memory_per_executor, which
would mean we wouldn't do anything than restarting the executors
without getting anything done and the amount of current executors
would not go under `min_executors_count`, politely terminate executor

4. the polite termination should be able to hand over all the tasks to
the other executors and once everything is finalized on the executor, it would just exit

5. the daemon monitor would notice the executor getting closed and running a new executor

It would be configurable, turned off by default (for development) but we would configure
this in production, where we can rely on the monitor being present.