Reject blocks with high precision timestamps.

This commit adds an additional sanity check to ensure the block that is
being processed does not contain a timestamp with a precision higher than
one second.  This is necessary because the consensus rules only deal with
whole seconds in the time comparisons whereas the internal data structures
make use of Go time.Time values which support up to nanosecond precision.

Also, add a test to ensure the new functionality works as expected.

ok @owainga