Modify the Aphlict client to use `LocalConnection`.

Summary:
Ref T4324. Currently, an Aphlict client (with a corresponding connection to the Aphlict Server) is created for every tab that a user has open. This significantly affects the scalability of Aphlict as a service. Instead, we can use `LocalConnection` instances to coordinate the communication of multiple Aphlict clients to the server.

Similar functionality existed prior to D2704, but was removed as the author was not able to get this functionality working as intended. It seems that the main issue with the initial attempt was the use of the `setTimeout` function, which seemed to be a blocking call which prevented messages from being received. I have instead used an event-based model using a `Timer` object.

Roughly this works as follows:

# The first instance will create an `AphlictClient` and an `AphlictMaster`. The `AphlictClient` will register itself with the `AphlictMaster` and will consequently be notified of incoming messages.
# The `AphlictClient` is then responsible for pinging the `AphlictMaster` at regular intervals. If the client does not ping the master in a given period of time, the master will assume that the client is dead and will remove the client from the pool.
# Similarly, the `AphlictMaster` is required to respond to pings with a "pong" response. The pong response lets the clients know that the `AphlictMaster` is still alive. If the clients do not receive a pong in a given period of time, then the clients will attempt to spawn a new master.

Test Plan: I have tested this on our Phabricator install with a few tabs opened and inspecting the console output. I will upload a screencast of my test results.

Reviewers: #blessed_reviewers, epriestley

Reviewed By: #blessed_reviewers, epriestley

Subscribers: epriestley, Korvin

Maniphest Tasks: T4324

Differential Revision: https://secure.phabricator.com/D9327