Fix return value check for anetTcpAccept

anetTcpAccept returns ANET_ERR, not AE_ERR.

This isn't a physical error since both ANET_ERR
and AE_ERR are -1, but better to be consistent.
Remove some redundant code

Function nodeIp2String in cluster.c is exactly
anetPeerToString with a pre-extracted fd.
Remove redundant IP length definition

REDIS_CLUSTER_IPLEN had the same value as
REDIS_IP_STR_LEN.  They were both #define'd
to the same INET6_ADDRSTRLEN.
Reset op_sec_last_sample_ops when reset requested

This value needs to be set to zero (in addition to
stat_numcommands) or else people may see
a negative operations per second count after they
run CONFIG RESETSTAT.

Fixes antirez/redis#1577
Merge branch 'unstable' of github.com:/antirez/redis into unstable