The format of tid assumes ~ ns precision, and it is only formatted to µs
precision by default. So don't truncate TimeStamp value when computing
it from Tid, and perform the µs-rounding only on formatting.

The float numbers are not always exactly as in python. For example the
following program

	tidv = [
	    0x0000000000000000,
	    0x0285cbac258bf266,
	    0x0285cbad27ae14e6,
	    0x037969f722a53488,
	    0x03b84285d71c57dd,
	    0x03caa84275fc1166,
	]

	for tid in tidv:
	    t = TimeStamp.TimeStamp(p64(tid))
	    print '0x%016x %s %.9f\t%.9f' % (tid, t, t.timeTime(), t.second())

prints:

	0x0000000000000000 1900-01-01 00:00:00.000000 -2208988800.000000000     0.000000000
	0x0285cbac258bf266 1979-01-03 21:00:08.800000 284245208.800000191       8.800000185
	0x0285cbad27ae14e6 1979-01-03 21:01:09.300001 284245269.300001621       9.300001496	<-- ex here
	0x037969f722a53488 2008-10-24 05:11:08.120000 1224825068.119999886      8.119999878
	0x03b84285d71c57dd 2016-07-01 09:41:50.416574 1467366110.416574001      50.416573989
	0x03caa84275fc1166 2018-10-01 16:34:27.652650 1538411667.652649879      27.652650112

the difference is due to floating point operation ordering, because
TimeStamp.timeTime() looses precision - e.g. for marked case:

	In [8]: '%.10f' % (281566860.000000000 + 9.300001496)
	Out[8]: '281566869.3000015020'

We don't try to mimic float64 behaviour to Python exactly - because it is even
different for PURE_PYTHON=y or C TimeStamp implementations. However we don't
limit due to that our timestamp precision to only 1µs.

In other words we keep on maintaining exact compatibility with Python on
printing, but timestamp values itself are now ~ ns precision.