Leaked TCP connection when connection to DB wasn't completed

[lexas2509]

Describe the bug
We observed leaked TCP connection when attempts to establish connection are unsuccesful

To Reproduce
n/a

Expected behavior
Underlying TCP connection released if database connection is failed to establish

Additional context
Similar to:

• Connection not closed when database name is incorrect #173 fix #224,
  -connection not closed when database name is incorrect #173

there are other case when TCP connection can be leaked, see below

If TCP connection was successfully created in dialConnection, but there is failure in succeeding action then TCP keeps open and effectively leaked.

Some code points where this connection can be leak:
573423d#diff-03d2022a494bc442893aa7389ad6f08e1003ef9d858972b0daec68b6c2950366L1203

573423d#diff-03d2022a494bc442893aa7389ad6f08e1003ef9d858972b0daec68b6c2950366L1229

573423d#diff-03d2022a494bc442893aa7389ad6f08e1003ef9d858972b0daec68b6c2950366L1234

and many others return nil, err effectively lead to leaked TCP connection.