logfile: add support for filesize based rotation

Extends the logfile plugin sntax to accept a key value string.
Supported values are:
- logfile, the actual file name of the log file (mandatory)
- backupname, the file name of the rotated log
- maxsize, the size in bytes that triggers rotation

maxsize is mandatory if you want rotation, if you omit backupname the logfile with a
timestamp appended would be used as file name.

Example:

logger = staticlogger file:logfile=%dstatic.log,backupname=%dstatic.log.old,maxsize=1500
log-route = staticlogger app: -1|req: -1

will create a file based logger called static logger that would log
all the static requests to a file called static.log, which would be
rotated each 1500 bytes to a file named static.log.old

Fixes #542