Stop HealthChecker after disconnect

[confususs]

Currently when you connect a 'healthChecker' is started on transport. That one is responsible for sending periodic OPTIONS requests to the sip server to ensure the connection is still alive.

Right now it is not stopped after a disconnect, causing clients that were online and then offline to still have their healthChecker active.

Version

0.2.9

File / Feature

src/transport.ts

Expected behaviour

The healthChecker should stop when disconnect is called.

Actual behaviour

The healthChecker continues when disconnect is called.

Other info

Added ddae5f9 but it broke tests, so reverted it before publishing a new version. The better way is to add it thruogh a PR