support setting default rules for resources

[linlinisme]

Does this pull request fix one issue?

Fixes #66

Describe how you did it

I adopt another design:
the default rules can be created and added in the resource pre-source init while application first time access resource.
I put this op in the old lookProcessChain method, now it rename to lookProcessChainAndInitDefaultRules.
The reason I do this as follows:

1. resource corresponding default rules can be created one time and only one time.
2. it's thread safe, if we do it at other place we may need extract syn op.
3. setting resource's default rules can be treated as a part of init method.
4. it's created on demand.

Describe how to verify it

run test cases.

Special notes for reviews

[pigercc]

if (SentinelConfig.isGlobalRuleOpen()) {
                for (Map.Entry<String, Set<DegradeRule>> entry : defaultDegradeRules.entrySet()) {
                    if (degradeRules.containsKey(entry.getKey())) {
                        defaultDegradeRules.remove(entry.getKey());
                    } else {
                        degradeRules.put(entry.getKey(), entry.getValue());
                    }
                }
            }

thanks for the code,it is helpful when i also need the global rule,but i think the code has a bug

the right expectation is:
if I set degrade rule, use mine
else use the global rule

but actual performance:

1. lookProcessChainAndInitDefaultRules init

 setRulesForResource(resource, degradeConf.get(resource));
defaultDegradeRules.put(resource, degradeConf.get(resource));

2 I set my degrade rule, is work
3 I remove my degrade rule,but now cannot apply the global rule

when remove happend,nacos handler will call
public void configUpdate(List<DegradeRule> conf)
and conf of course not cantains the global rule

and in the for code,defaultDegradeRules has remove resource

 for (Map.Entry<String, Set<DegradeRule>> entry : defaultDegradeRules.entrySet())

so degradeRules cannot add the global rule back

degradeRules.put(entry.getKey(), entry.getValue());