Fix another FileBufferedChannel mbuf destruction bug
Fix another FileBufferedChannel mbuf destruction bug

ReadContext also contains an mbuf, so it must also be
destroyed only on the event loop thread.

kewaunited/passenger,phusion/passenger,kewaunited/passenger,kewaunited/passenger,phusion/passenger,phusion/passenger,cgvarela/passenger,phusion/passenger,kewaunited/passenger,antek-drzewiecki/passenger,clemensg/passenger,phusion/passenger,clemensg/passenger,kewaunited/passenger,antek-drzewiecki/passenger,antek-drzewiecki/passenger,cgvarela/passenger,cgvarela/passenger,cgvarela/passenger,cgvarela/passenger,clemensg/passenger,antek-drzewiecki/passenger,antek-drzewiecki/passenger,antek-drzewiecki/passenger,phusion/passenger,clemensg/passenger,phusion/passenger,antek-drzewiecki/passenger,kewaunited/passenger,kewaunited/passenger,cgvarela/passenger,antek-drzewiecki/passenger,phusion/passenger,clemensg/passenger,cgvarela/passenger,cgvarela/passenger,clemensg/passenger,clemensg/passenger,kewaunited/passenger,clemensg/passenger