lib-index: Fix cache lookups from uncommitted transactions with in-memory indexes

lib-index: Fix cache lookups from uncommitted transactions with in-memory indexes

This especially fixes mail_get_header_stream() with imapc and
imapc_features=fetch-headers, because it works by first fetching the
specific headers and putting them to the cache transaction, then later on
relying on index_mail_get_header_stream() to get them from the transaction.
Before this fix they wouldn't be found, which triggered another unnecessary
FETCH BODY.PEEK[HEADER].