SOLR-18083: Fix replication and other general operations in readOnly mode

[HoustonPutman]

https://issues.apache.org/jira/browse/SOLR-18083

Issues with readOnly:

• Currently parts of recovery issue a commit to the leader, which can't happen if the leader is in readOnly mode. Since readOnly replicas will have already committed, we can introduce an option for commit to be a NO-OP if the core is in read-only mode.
• If we are in read-only mode, we do not need to wait for documents to come in during various operations
• IndexWriter cannot be opened in readOnly mode. The issue here is that certain operations open an IndexWriter for non-write reasons. Such as metrics, or opening a SolrSearcher with an IndexWriter to tell it what to read.

One other issue during indexFetching a readOnly core is that if we want to delete all documents during a fetch, then we want to be able to open an indexWriter to clear the index. While this goes against the spirit of a readOnly core, we are replicating from the leader and want to be in-sync. So we should allow writing in this case.

So instead of just disallowing opening IndexWriters at all, we should allow opening an indexWriter with a flag that says it will be used in accordance with the spirit of read-only rules.