getinfo: add microsecond precise timers for various intervals

Provide a set of new timers that return the time intervals using integer
number of microseconds instead of floats.

The new info names are as following:

CURLINFO_APPCONNECT_TIME_T
CURLINFO_CONNECT_TIME_T
CURLINFO_NAMELOOKUP_TIME_T
CURLINFO_PRETRANSFER_TIME_T
CURLINFO_REDIRECT_TIME_T
CURLINFO_STARTTRANSFER_TIME_T
CURLINFO_TOTAL_TIME_T

Closes #2495