Shared storage online split support

The strategy for online shard splitting is the same as that for normal (non SHARED shards.)
During a split, the leader will forward updates to sub-shard leaders, those updates will be buffered by the transaction log while the split is in progress, and then the buffered updates are replayed.

One change that was added was to push the local index to blob store after buffered updates are applied (but before it is marked as ACTIVE):
See https://github.com/apache/lucene-solr/commit/fe17c813f5fe6773c0527f639b9e5c598b98c7d4#diff-081b7c2242d674bb175b41b6afc21663

This issue is about adding tests and ensuring that online shard splitting (while updates are flowing) works reliably.