sheep: make sheep request stat finer

With this patch we can poke more information about the node stat:

$ dog node stat -w

...
Client	12	340681	100459	233694	0	6528	44 GB	26 GB	95 MB	0.0 MB
Peer	6	41013	4008	37005	0	0	8.6 GB	3.9 GB	23 MB	0.0 MB
Request	Active	Total	Write	Read	Remove	Flush	All WR	All RD	WRBW	RDBW
Client	5	341362	100459	234375	0	6528	44 GB	26 GB	98 MB	0.0 MB
Peer	0	41128	4008	37120	0	0	8.6 GB	3.9 GB	24 MB	0.0 MB
Request	Active	Total	Write	Read	Remove	Flush	All WR	All RD	WRBW	RDBW
Client	0	341969	100459	234982	0	6528	44 GB	26 GB	92 MB	0.0 MB
Peer	0	41220	4008	37212	0	0	8.6 GB	3.9 GB	24 MB	0.0 MB
Request	Active	Total	Write	Read	Remove	Flush	All WR	All RD	WRBW	RDBW
Client	8	342584	100459	235597	0	6528	44 GB	26 GB	113 MB	0.0 MB
Peer	0	41335	4008	37327	0	0	8.6 GB	3.9 GB	23 MB	0.0 MB
Request	Active	Total	Write	Read	Remove	Flush	All WR	All RD	WRBW	RDBW
Client	6	343069	100460	236081	0	6528	44 GB	26 GB	86 MB	4.0 MB
Peer	2	41429	4009	37420	0	0	8.7 GB	3.9 GB	26 MB	1.0 MB
...

Reviewd-by: Hitoshi Mitake <mitake.hitoshi@lab.ntt.co.jp>
Signed-off-by: Liu Yuan <namei.unix@gmail.com>
sheep: make passing log level uniform for sheep and dog

With this patch, we all set/get level with string instead of cryptical number

Reviewed-by: Hitoshi Mitake <mitake.hitoshi@lab.ntt.co.jp>
Signed-off-by: Liu Yuan <namei.unix@gmail.com>