Adding sub second timestamps and request end time to mod_log_config.

Add special format tokens to %{...}t. The extended syntax allows the
form: "WHICH:WHAT".

WHICH is either:
- "begin": use the time when the request started
- "end": take "now" as the time
You can omit WHICH, default is "begin".
If you omit WHICH, the separating column is not allowed.

WHAT is either:
- "sec": timestamp in Unix seconds
- "msec": timestamp in Unix milliseconds
- "msec_frac": millisecond fraction of the Unix timestamp,
               3 digits, 0-padded
- "usec": timestamp in Unix microseconds
- "usec_frac": microsecond fraction of the Unix timestamp
               6 digits, 0-padded
- anything different from those tokens: use strftime()
You can omit WHAT, default is the formatted timestamp as
used by the Common Log Format.

The implementation uses a new request_config for mod_log_config
to pass the request end time around between different calls to
log formatters, but the end time is only generated if needed.

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@979120 13f79535-47bb-0310-9956-ffa450edef68