Make ackDeadline editable

[stephenplusplus]

This is a request originally from @mkamioner in this PR.

Love the new changes, but I miss the ability to specify my ackDeadline -- Sometimes I have processes with long running jobs and I want to be able to change it in once place

[baronfel]

Agree on this. We have long-running processes that we dynamically refresh the ackDeadline on a timer based on a rolling average of job completion, and we lost this capability on this version.

[rossj]

I also have long-running jobs, and the current approach will start with an ackDeadline of 10 seconds and renew this deadline at a random interval somewhere between 0 and 0.9 times the current deadline (0-9 seconds initially). This is too frequent for my taste, so for now I've overridden the starting ackDeadline and the histogram object to maintain a constant ackDeadline of 5 minutes.

const sub = client.subscription(name);
sub.ackDeadline = 5 * 60 * 1000;
sub.histogram = {
    add: () => { },
    percentile: () => sub.ackDeadline
};

One can further monkey patch the subscription's setLeaseTimeout_ function in order to change the lease renewal timeout as desired. For instance, I've set it to be a constant 4 minute renewal timeout.

[callmehiphop]

We should have a release coming up in the next several days that will include the ability to manually set ack deadlines.

const subscription = topic.subscription('my-sub', {ackDeadline: 20}); // 20 seconds