firebase-ios-sdk

Add a schema migration that drops the query cache (#1558)

This is a force fix for potential existence filter mismatches caused by
firebase/firebase-ios-sdk#1548

The essential problem is that when resuming a query, the server is
allowed to forget deletes. If the number of incorrectly synthesized
deletes matches the number of server-forgotten deletes then the
existence filter can give a false positive, preventing the cache from
self healing.

Dropping the query cache clears any client-side resume token which
prevents a false positive existence filter mismatch.

Note that the remote document cache and mutation queues are unaffected
so any cached documents that do exist will still work while offline

https://github.com/firebase/firebase-js-sdk/pull/1019