optimize mapper builder

[mawen12]

Hi, I have found the XMLMapperBuilder contains two points can be optimized.

First: use method instead of parent variable

private void cacheElement(XNode context) {
  ...
  Class<? extends Cache> typeClass = typeAliasRegistry.resolveAlias(type);
  ...
  Class<? extends Cache> evictionClass = typeAliasRegistry.resolveAlias(eviction);
  ...
}

Its superclass is BaseBuilder which provide method resolveAlias(alias) to do this, may be can use method directly.

Second: use lambda to lazy compute

private Discriminator processDiscriminatorElement(XNode context, Class<?> resultType, List<ResultMapping> resultMappings) {
  ...
  String resultMap = caseChild.getStringAttribute("resultMap", processNestedResultMappings(caseChild, resultMappings, resultType));
  ...
}

The method getStringAttribute(value, default) return default only when value is null.
So when value is not null, default will be ignored,
By use lambda, default will be computed only when value is null.

[coveralls]

coverage: 87.068%. remained the same
when pulling 9bfbfe4 on mawen12:optimize-mapper-builder
into 2967694 on mybatis:master.