Uses a global variable to carry state beyond the JITed boundry for now

Also add some changes from the previous code review.

Refactors much of the multi-wasm-runtime code to remove duplication and global accessors. WAVM is currently non functional because of some problems with passing through the non-global apply_context

(not wired up yet)

When wavm interface was refactored sbrk initialization was placed in the wrong spot. Due to shared memory, sbrk init needs to go after resetMemory() call