Update circumvention map in Iran and provide snowflake as first option

circumvention map:"ir":{"settings":[{"bridges":{"type":"obfs4","source":"bridgedb"}},{"bridges":{"type":"snowflake","source":"builtin"}}]}

Snowflake seems to work quite well in Iran according to OONI probe tests, while bridgeDB could be distributing blocked bridges.

So, should we change the priority here? Snowflake first, and if it fails, bridgeDB bridges? (cc: @meskio)