LP1008334 : Speedup specific datetime queries that got slower with introduction of microseconds in 5.3

- Item::get_seconds() now skips decimal arithmetic, if decimals is 0. This significantly speeds up from_unixtime() if no fractional part is passed.
- replace sprintfs used to format temporal values  by hand-coded formatting 
  
Query1 (original query in the bug report)
BENCHMARK(10000000,DATE_SUB(FROM_UNIXTIME(RAND() * 2147483648), INTERVAL (FLOOR(1 + RAND() * 365)) DAY)) 
  
Query2 (Variation of query1 that does not use fractional part in FROM_UNIXTIME parameter)
BENCHMARK(10000000,DATE_SUB(FROM_UNIXTIME(FLOOR(RAND() * 2147483648)), INTERVAL (FLOOR(1 + RAND() * 365)) DAY)) 
  
Prior to the patch, the runtimes were (32 bit compilation/AMD machine)
Query1: 41.53 sec 
Query2: 23.90 sec
  
With the patch, the runtimes are
Query1: 32.32 sec (speed up due to removing sprintf)
Query2: 12.06 sec (speed up due to skipping decimal arithmetic)