Do not create/use/cache execution context per contract

[linh2931]

A part of eosnetworkfoundation/product#149.

Currently an execution context is created/used/cached per contract. Towards our goal of sharing the same compiled WASM code across all the threads, we need to make an execution context per thread (or wasm interface). A good side effect of this is memory usage reduction since execution context per contract is not cached any more in physical memory in running nodeos.