Added option to write HTTP request logs to separate file

[stuartcarnie]

Added a new option for the [http] service configuration to redirect request logging to a separate file, to address part of #9438

Add a way to split out HTTP logs into a separate access log.

The new key is as follows

[http]
  # When HTTP request logging is enabled, this option specifies the path where 
  # log entries should be written. If unspecified, the default is to write to stderr, which
  # intermingles HTTP logs with internal InfluxDB logging.
  #
  # If influxd is unable to access the specified path, it will log an error and fall back to writing
  # the request log to stderr.
  # access-log-path = ""

NOTES:

• when request logging is redirected to a file, the [httpd] prefix is stripped, so access log parsing tools like lnav can render them without additional modification.
• to rotate the log file, use the copytruncate method of logrotate or similar to leave the original file in place.

[aanthony1243]

LGTM

[stuartcarnie]

I improved the config help