There are some cases where there is a complex hosting strategy and folks want a readonly database and are hosting on Azure. In that case, it is not entirely possible to have a readonly Umbraco database because SqlMainDom is required and part of that requirement is to have read/write access to the umbraco key value table.
This PR allows for the default MainDom to be replaced and to allow for an SqlMainDomLock to use an alternate connection string so that a separate read/write database can be used.
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