Today if the primary discovers that an indexing request needs a mapping update
then it will send it to the master for validation and processing. If, however,
the put-mapping request is invalid then the master still processes it as a
(no-op) cluster state update. When there are a large number of indexing
operations that result in invalid mapping updates this can overwhelm the
master.

However, the primary already has a reasonably up-to-date mapping against which
it can check the (approximate) validity of the put-mapping request before
sending it to the master. For instance it is not possible to remove fields in a
mapping update, so if the primary detects that a mapping update will exceed the
fields limit then it can reject it itself and avoid bothering the master.

This commit adds a pre-flight check to the mapping update path so that the
primary can discard obviously-invalid put-mapping requests itself.

Fixes elastic#35564

This reverts commit 148206d.