Rename and rework get_indexer function

[UlrichB22]

Fixes #1017.

• The function get_indexer() becomes get_doc().
• A new parameter retry is added to get_doc().
• The retry and timeout in get_doc() only apply to calls from store_revision. At the end of this method, the revision that has just been written to the backend is read again. This ensures that this revision exists. In all other cases, e.g., when calling with an invalid, non-existent revid, get_doc immediately triggers a KeyError.

[roland-ruedenauer]

Renaming this function is a good idea.

Some additional suggestions:

• why not be more verbose and rename it into get_document
• instead of passing in the magic fn argument pass in the indexer (instance of IndexingMiddleware) and call the required indexer method
• use time.monotonic() instead of time.time()
• change the code comment "Return a valid indexer document ..."

[UlrichB22]

@roland-ruedenauer, 🙏 thanks for your valuable suggestions and your help.