Don't use fielddataBuilder to test for aggregatability

[romseygeek]

The current default implementation of isAggregatable on MappedFieldType tries to construct a field data builder, and returns true or false depending on whether an exception was thrown during construction. This is fairly fragile, and is becoming increasingly so with the introduction of field data contexts, so that a non-aggregatable field type may in fact provide field data to scripts.

This commit changes the default implementation to check for docvalues instead of directly building a fielddata builder, and adds checks to MapperTestCase that verify these implementations work correctly.

[elasticsearchmachine]

Pinging @elastic/es-search (Team:Search)

[nik9000]

Well. Jenkins is panicing a bit. So maybe not that easy.

[romseygeek]

@elasticmachine run elasticsearch-ci/part-1

[romseygeek]

@elasticmachine update branch