refactor stmt_store
  * main ets table has one entry per statement
  * row caches in separate ets tables
  * delete statement info on finish
  * fix tests (arbitrary keys in stmt store no longer allowed)
work on reducing noise with temp db
fix broken tests; even more noise reduction
account for already_started in erbi:start/0
remove "shadowed" warning
replace amazingly slow ets:select with tab2list
add special 'final' statement handle for drivers
Merge pull request #12 from qhool/stmt_store_refactor

Refactor stmt store for enhanced performance