Don't continuously reparse tiles

[kkaefer]

When there are assets (like glyph PBFs) missing, we're currently continually reparsing the tiles, only to end up finding that we still need an asset, which we already know after the first parse attempt. This uses too much CPU resources. Instead, we should be smart about resuming tile parsing once all of the required assets for a layer came in.

[jfirebaugh]

This was fixed by #8341.