Add utility functions for determine the type of a dbpage
Add TransactionInfo to Environ to note the existing txns
Prototype for creating transactions
Make the code compilable to rust-0.12-prev
Change the native_thread module to be thread_local_storage module
Create a native_thread module to get the thread_id
Add readers & writers mutex
Change MappedRegion's implementation with the standard lib's MemoryMap
Add getter/seeter to TransactionInfo