Use `LuaActor` with `SyncArbiter`

[poga]

SyncArbiter allows us to put blocking statements to separated threads.

LuaActor is implemented with AsyncContext. In order to start an actor with SyncArbiter, it has to with SyncContext.

However, SyncContext does not provides rich APIs from AsyncContext, therefore it's not really compatible between these two implementation.

Maybe we can provide an alternative implementation such as SyncLuaActor?