kvstore/cache_kv.go中很多方法没有使用defer的方式来释放锁，可能会有死锁的风险

[colin-si]

Describe the bug
kvstore/cache_kv.go中很多方法没有使用defer的方式来释放锁，可能会有死锁的风险

To Reproduce
如下代码可能 在获取锁之后若有执行异常则会崩掉，则全局锁则无法释放，造成死锁。

https://github.com/apache/servicecomb-service-center/blob/ce7afe04f5bcad594bede6204a37443bfa9c0a00/datasource/etcd/state/kvstore/cache_kv.go#L45C6-L45C6

servicecomb-service-center/datasource/etcd/state/kvstore/cache_kv.go

Line 55 in ce7afe0

c.rwMux.RUnlock()

servicecomb-service-center/datasource/etcd/state/kvstore/cache_kv.go

Line 62 in ce7afe0

c.rwMux.RUnlock()

servicecomb-service-center/datasource/etcd/state/kvstore/cache_kv.go

Line 69 in ce7afe0

c.rwMux.RUnlock()

servicecomb-service-center/datasource/etcd/state/kvstore/cache_kv.go

Line 76 in ce7afe0

c.rwMux.Unlock()

servicecomb-service-center/datasource/etcd/state/kvstore/cache_kv.go

Line 82 in ce7afe0

c.rwMux.Unlock()

servicecomb-service-center/datasource/etcd/state/kvstore/cache_kv.go

Line 95 in ce7afe0

c.rwMux.Unlock()

Expected behavior
此处预期获取锁之后的执行逻辑无异常，及时把锁释放掉。
为了增强可靠性，可以通过defer的方式来释放锁。

Platform And Runtime (please complete the following information):

Platform

• OS: [e.g. iOS]
• Browser [e.g. chrome, safari]
• Version [e.g. 22]

Runtime

• Version [e.g. Go 1.11]

Additional context
Add any other context about the problem here.