org.apache.avro.util.Utf8 bug in `hashCode`

Bug : utf8 hash reset is not correctly done, as a result, returns different hash when the values are logically equal.

cause commit : 

 https://github.com/apache/avro/commit/d5d1cd70e920f45468bf696d3668b496a9adbab3#diff-7001731cdafe4fc24f86e71c7e8a0b61

https://github.com/apache/avro/commit/8e345c48e8f9708c27af63754965777314dac85c#diff-7001731cdafe4fc24f86e71c7e8a0b61

reproduce : 

 org.apache.avro.util.Utf8 utf8 = new org.apache.avro.util.Utf8();
 utf8.set("hello");
 System.out.println(utf8.hashCode()); // 99162322
 utf8.set("hello");
 System.out.println(utf8.hashCode()); // -1538739392

Suggested fix.

1. get rid of `hasHash` field. In my opinion, it leads to more bug-prone by adding additional variable to take care of. `hash != 0` should be enough to check the presence of hash. Redundant computation for `0 hash` elements should be negligible.
2. reset `hash = 0` on every mutation call.
3. not needed, but I want to see some minor optimization like avoiding memory access by creating local variable (int hash = 0; .... this.hash = hash).