fix(perf): reuse port-forwards when hot-reloading

[thsig]

Added a simple in-process cache of port-forwards allocated for syncing
during hot-reloading.

This results in dramatic (often 50%) speedups for hot reloads subsequent
to the initial one, during the execution of the enclosing Garden command.

Addresses #458.

[eysi09]

What does the speedup translate to in terms of milliseconds, for, say, the vote example?

[thsig]

I did a couple of runs in the vote project, with the vote service configured for hot reloading (stopping and then re-running garden dev --hot-reload=vote after the first run). Here are the numbers:

First run

• Initial hot reload: 335ms.
• Next five hot reloads: 201ms, 206ms, 145ms, 216ms, 228ms (average: 119ms).
• Average speedup compared to initial: 64%.

Second run

• Initial hot reload: 327ms.
• Next five hot reloads: 154ms, 142ms, 158ms, 229ms, 215ms (average: 179ms).
• Average speedup compared to initial: 45%.

I got similar distributions and speedups in the gatsby-hot-reload example project, but a slightly larger speedup on average.

[edvald]

Just one comment, otherwise great!