Fix SIGUP Memory Leak

[okimiko]

This addresses #585 and may address #1383.

I stepped down the code and found out, that the memory issue does not apply to the light build. After some debugging I found

tileserver-gl/src/serve_rendered.js

Line 1060 in a245126

const renderer = new mlgl.Map({

and

tileserver-gl/src/serve_rendered.js

Line 1220 in a245126

return new advancedPool.Pool({

I assumed that these are not found by the garbage collector because there is a reference somewhere, to I added an explicit cleanup function clear, which required the before inaccessible serving to handle the repo.

As a result I have no more increasing memory with reloading by SIGHUP.