page_service: batching needless waits for unbatchable requests

[problame]

          Consider the following request sequence: `get_page (lsn=x), get_page (lsn=x), db_size`.

First call to read_batch_from_connection: Return batch with get page requests and carry contains db_size request.
Second call to read_batch_from_connection: Wait until the batching time-out or the next request to serve the db_size.

Same applies for sequence: db_size.

We only care about batching get_page requests, so this is not necessary. Feels like we could work around without too much effort.

Originally posted by @VladLazar in #9792 (comment)