Flag for disabling microseconds time resolution support

[josephbuchma]

This change allows to restore v1.2 behavior of
dealing with time.Time values.

When connection is open with disableMilliseconds=true
time.Time values will be rounded DOWN to second resolution
before insert into database.

Related issue #713

Checklist

• Code compiles correctly
• Created tests which fail without the change (if possible)
• All tests passing
• Extended the README / documentation, if necessary
• Added myself / the copyright holder to the AUTHORS file

[methane]

DATETIME(6) keeps micro seconds, not milli.
So option name should be subsecond instead of millisecond.

But as I wrote in #713, I'm -1 on adding option until I understand how it's important.

[julienschmidt]

@josephbuchma Can you elaborate when the behavior this flag provides is useful?

[methane]

It's too late to "for v1.2 compatibility".
User should manually round down if they need.