We cannot rely on in-place --reindex to handle thread_id
changes when we fix threading bugs in the search indexer
like in commit 83425ef12e4b65cdcecd11ddcb38175d4a91d5a0
("searchidx: deal with empty In-Reply-To and References headers")So, bump the schema version and pay the cost of requiring extra disk space to create a new index in parallel.
