fix(subscriber): fix memory leak from historical PollOps

[hawkw]

Motivation

Currently, console-subscriber retains all PollOps that have been
recorded since the application started, and sends all of them in the
first resource update for a new subscription.

I don't think this is correct, for a few reasons. It's necessary to send
all currently live tasks and resources to new subscriptions, because
future events may reference those tasks or resources. This isn't the
case for PollOps --- unlike tasks, resources, and even async ops, a
PollOp doesn't represent an object, it represents an event, a
single time an object was polled. There's no reason to send all previous
poll ops in the first update for a new subscription.

Storing all the poll ops that have ever occurred results in a memory
leak (see #256); unlike other tracked entities, we never clear anything
out of the vector of all poll ops that have ever occurred in the
program.

Solution

This branch just removes the all_poll_ops Vec entirely, and changes
the subscriber to only send new poll ops in the first update.

We could change this so that poll ops are associated with a timestamp,
and older ones are cleared out of a vec of all poll ops when they time
out. However, the tokio-console CLI doesn't currently have a way of
showing historical poll ops anyway, so there's no reason to keep this
data. If that changes, we can put them back, but for now, the simplest
solution is to just remove this.

Fixes #256

[hawkw]

cc @zaharidichev

[zaharidichev]

LGTM. Sorry I could not get to that in time.