MINOR: log: Add logurilen tunable.

MINOR: log: Add logurilen tunable.

The default len of request uri in log messages is 1024. In some use
cases, you need to keep the long trail of GET parameters. The only
way to increase this len is to recompile with DEFINE=-DREQURI_LEN=2048.

This commit introduces a tune.http.logurilen configuration directive,
allowing to tune this at runtime.