Add a way to get a read only connection

[davecramer]

r2dbc-spi/r2dbc-spi/src/main/java/io/r2dbc/spi/ConnectionFactory.java

Line 24 in ad3ffd5

public interface ConnectionFactory {

Setting mutability on an existing connection is way too late IMO to get a read only connection. This brings up another point which is how to provide multiple connections. PostgreSQL JDBC has a notion of providing multiple hosts/ ports in the URL but no notion of which ones are read only. This might be better handled with a custom connection factory, but I still think setting mutability on the transaction is too late.

[gregturn]

Closed based on #14.