Add oraclejdk8 to Travis CI.
Reduce the amount of time spent creating record types that already exist.

The key in the type cache used to be the type (basically, its digest) and is now a pair consisting of the field names and types. The type factory checks for the type using a quick temporary key, then makes a permanent key if the type does not seem to exist. RelDataTypeFactory.createStructType(List<RelDataType>, List<String>) is now the best API to use, if speed is important; FieldInfoBuilder is fairly efficient too.