Optimized NpgsqlConnection new and Clone()

new NpgsqlConnection(string) is slow.
Redirect clone to new private constructor which uses NpgsqlConnectionStringBuilder instead of string.
Since DbConnectionStringBuilder Clear is slow too, reuse NpgsqlConnectionStringBuilder.
NpgsqlConnectionStringBuilder can change in ChangeDatabase, so Clone it there.

Optimized LogConnectionString()
Skip looping if logging is disabled.