Not documented which DBs are parallel-safe

[oliverdain]

Describe the Bug

The FAQ contains a helpful section titled, "What happens if two programs try and update the database at the same time?". It says this is safe/supported for some databases but I can't find a list anywhere of which are and are not supported. For example, I'm using Cloud Spanner but the docs for it don't specify if it's supported or not.

I took a quick look at the code and I see comments like, Lock implements database.Driver but doesn't do anything because Spanner only enqueues the UpdateDatabaseDdlRequest but then the ensureVersionTable method appears to aquire a lock so I'm a bit confused.

Steps to Reproduce

Pick a DB and try to figure out if it supports parallel migrations or not. At least in some cases it's impossible without reading the code.

Expected Behavior

I suggest either:

1. In the main README where you list all the databases supported turn that into a table with a column titled "parallel safe" or similar.
2. In the README for each driver clearly indicate what the driver does or does not support.

Migrate Version

current HEAD