Semantic iterator for all objects that we store in merk needed

Nobody has claimed this yet.

This may already be compulsory to finish the hash migration for merk keys, but it does not appear that there is a single iterator that iterates over all of the objects that we currently store in merk. The existing iterators (e.g. events, symbols, multisig) appear to have been constructed in bespoke fashion as they've been needed. It would be clearer for developers working on future migrations, where the items to be modified may be different from data modified in past migrations, to work with a more general iterator and filter from it, versus constructing individual iterators for each item type.

Start by locating the existing bespoke merk iterators for events, symbols, and multisig, then compare how each enumerates stored objects. Define the shared iterator and its filtering boundary from those implementations. Done means future migrations can iterate over all stored objects and select the relevant items without adding another bespoke iterator.

Written by the indexing model from the issue text.