doc: added performance notes to the sections for ngx.var and ngx.ctx.
ensured that the resolver handler will not call into the dead request context.
fixed the test plan in 058-tcp-socket.t.
bugfix: fixed an incorrect error message. thanks doujiang for the patch in #373.
bugfix: the coctx cleanup handler might not be called before being overidden by other operations. this could happen when failing to yield in an error handler (for xpcall).
resolved a conflict while merging with master.