make memfile thread safe

(Changed the description)

memfile is not thread safe for two things:

• the multi index table (read / write possible conflicts)
• the backup file (simultaneous writes)

As the code waits for changes to be committed to the backup file before applying them to the multi index table in fact both should be addressed the same way. Note that C++11 has no RW locks (shared mutex in C++14) we should just use a simple manager mutex.