Added check for size that extends past the end of the store
Fix bug in threaded test causing cursor to try to seek to offset zero
Actually free resources in the mmap store destroy function
Added flag to decide what to do if a file already exists for a store being created
Added function to get a cursor the first data location in a store
Change store classes to new concurrency model

The semantics are as follows:

1. Multiple writers are allowed
2. After a sync begins, no further writers are allowed
3. After a sync completes, multiple readers are allowed
Added ability to reopen persistent store