Storm could usefully support readonly Stores

In a mutithreaded environment using SQLite, it can be useful to have a policy that only one Store at a time may ever be written to, to avoid getting "deadlocks" (which are eventually resolved by a timeout leading to a "database is locked" exception).

When implementing such a policy, it would be useful to be able to mark any store which isn't intended to be written to as readonly, to avoid coding errors. Such a store would raise an exception (probably a ProgrammingError) whenever an attempt to change the data in the database was made (eg, by adding a document, changing a property in an object retrieved from the database, or by executing some SQL which changes the database directly). Marking stores readonly in such a way would mean that an accidental coding error would result in a repeatable ProgrammingError, rather than an occasional (ie, thread execution order dependent) OperationalError("database is locked"). It could also help lead to clearer code.

To get a readonly store, a "readonly=1" option would be added to the URI used to open the database - all stores derived from that database would then be readonly.