Fix #387: fiber local storage in Lua
Remove cookie from fiber
Implement C API for fiber local storage

This patch removes `session` and `lua_storage` fields from struct fiber.
A new API for fiber-local variables is now used for this purpose.
This change solves dependency problems between session and fiber modules.
Review fixes for fiber local storage
Merge remote-tracking branch 'origin/master' into gh-387-lua-fiber-storage
Allocate transaction on_stop trigger with struct txn

Fix potential bug multiple parallel transactions