Move connection close statement to debug level

[arina-ielchiieva]

When closing connection, closing information is displayed at output level.

apache drill> !quit
Closing: org.apache.drill.jdbc.impl.DrillConnectionImpl

I am not sure if this information is useful for the user and has any value.
This ticket suggests to move connection related information to debug level.
Why info level is not suggested?
Because if we use info level such information still be displayed unless !set silent true is set. But when setting !set silent true, other more useful information will be hidden, for example, number of rows displayed and elapsed time 1 row selected (1.778 seconds) thus suggested level is debug.

[snuyanzin]

Merged as a63de2a

Thanks, @arina-ielchiieva