refactor init lock
refactor init lock

Summary:
`MainInit.go` is not a very descriptive name. also, the `MainInit`
module only hides two one-line functions that would be simpler if they were
inlined. So, renamed to `with_init_lock` and moved it to `ServerMain`.

Reviewed By: gabelevi

Differential Revision: D4900381

fbshipit-source-id: 98e8c6d13c3fafce871c8ed2bd457beaed965440

jamesgpearce/flow,JonathanUsername/flow,AgentME/flow,TiddoLangerak/flow,jamesgpearce/flow,claudiopro/flow,ylu1317/flow,samwgoldman/flow,jamesgpearce/flow,gabelevi/flow,popham/flow,AgentME/flow,fletcherw/flow,nmote/flow,ylu1317/flow,ylu1317/flow,JonathanUsername/flow,gabelevi/flow,MichaelDeBoey/flow,TiddoLangerak/flow,JonathanUsername/flow,facebook/flow,jamesgpearce/flow,claudiopro/flow,facebook/flow,claudiopro/flow,MichaelDeBoey/flow,samwgoldman/flow,jamesgpearce/flow,nmote/flow,TiddoLangerak/flow,jamesgpearce/flow,popham/flow,fletcherw/flow,mroch/flow,AgentME/flow,AgentME/flow,facebook/flow,gabelevi/flow,nmote/flow,JonathanUsername/flow,samwgoldman/flow,gabelevi/flow,samwgoldman/flow,ylu1317/flow,TiddoLangerak/flow,mroch/flow,popham/flow,fletcherw/flow,nmote/flow,nmote/flow,samwgoldman/flow,JonathanUsername/flow,mroch/flow,JonathanUsername/flow,claudiopro/flow,popham/flow,JonathanUsername/flow,facebook/flow,fletcherw/flow,facebook/flow,popham/flow,samwgoldman/flow,samwgoldman/flow,gabelevi/flow,nmote/flow,gabelevi/flow,mroch/flow,mroch/flow,facebook/flow,fletcherw/flow,mroch/flow,gabelevi/flow,popham/flow,MichaelDeBoey/flow,TiddoLangerak/flow,popham/flow,jamesgpearce/flow,MichaelDeBoey/flow,AgentME/flow,facebook/flow,claudiopro/flow,fletcherw/flow,nmote/flow,ylu1317/flow,MichaelDeBoey/flow,mroch/flow