Setting search_path in startup packet

As pointed out by @sehrope on PR #156, setting the ```search_path``` can be done directly in the startup packet when using v3 protocol, saving a round-trip.
Dealing with ```currentSchema``` is now done this way.

On v2 protocol, it keeps being set in an sql query after the connection has been made.

Note : I would consider merging PR #207 at the same time because I expect special characters to be now taken in account that way. And I think the behavior of ```currentSchema``` and ```setSchema()``` should remain consistent.