asynchronously load ctags
asynchronously load ctags

Summary: on a large php codebase, this could take loading time from
~11.2s to ~8.7s.  why you would ever let your codebase bloat to 8.7s
load is of course another question.

Reviewed By: marcel

Test Plan: time phpsh on all_good.pht, three runs each on old and new

Revert Plan: ok

rhee/phpsh,facebookarchive/phpsh,AsafMah/phpsh,terrorbox/phpsh,AsafMah/phpsh,AsafMah/phpsh,facebookarchive/phpsh,terrorbox/phpsh,terrorbox/phpsh,rjmi004/phpsh,rjmi004/phpsh,chooaya/phpsh,rjmi004/phpsh,rhee/phpsh,chooaya/phpsh,rhee/phpsh,chooaya/phpsh