Cfg: can't address implicit default group in sched_http_rules

[vankoven]

It is impossible to use default server group in sched_http_rules if it is not explicitly defined in config file.

E.g. the next config cannot be parsed correctly:

sched hash;
server 192.168.122.1:8000 conns_n=1;

srv_group backup {
sched hash;
server 192.168.122.1:8001 conns_n=1;

}
cache 0;

sched_http_rules {
  match default * * * backup=backup;
}

Tempesta log:

[  124.516470] [tempesta] ERROR: sched_http: srv_group is not found: 'default'
[  124.517535] [tempesta] ERROR: configuration parsing error
[  124.518707] [tempesta] ERROR: failed to start modules

Parsing of sched_http_rules section requires defining server groups before using them. But implicit default group will be created only when configuration file will be fully parsed (see tfw_sock_srv_start() function). This breaks user experience, especially if user writes full configuration of implicit default group before sched_http_rules section.

Posiible solution: create implicit default server group when the first server of that group was encountered.

[krizhanovsky]

The proposal should work. It'd be expected if we require C-behavior: define something before using it. So if a user uses default group in sched_http_rules and next defines the default group, then we should fail to error because we already created the goup when faced it in the scheduler sched_http_rules section.

[krizhanovsky]

Relates #51 and should be fixed in the same PR.