[TEST] decreased number of replicas to 0 in refresh tests

Some tests disable refresh and verify that documents cannot be found without an explicit refresh. With at least one replica and waiting for yellow though, it can happen that we start indexing docs when the replicas are not ready yet, and as soon as they become ready they get refreshed, causing the newly added docs to be unexpectedly found. Solution is to disable replicas for these specific tests.
Document http.cors-settings
Search template: Fix support for source parameter

If a search template was created using the source parameter, the
content of the parameter was put as source instead of sourceTemplate

Fixes #5556
Refactor common code for unmapped aggregators into `NonCollectingAggregator`.

Our aggregators that are dedicated to unmapped fields all look more or less
the same so this hopefully helps remove some spaghetti code.

Close #5528
[TEST] Test sending the request body as source parameter with GET method through REST tests

For around 10% of the calls that support the GET method, the request body gets now sent as `source` parameter with GET method (no randomized method in this case).

This way we can find bugs like #5556 (missing support for source param).
Fix some warnings reported by Findbugs.

Although high-severity bugs were mostly static variables that were not final,
it also found potential NPEs and bugs like `x ^= x;` or equality checks on
objects that don't share a common interface.

Close #5571
check "store" for binary mapper and check "index_name" for all mappers

closes #5474