MB-23719: Add dynamic change of ephemeral_metadata_purge_age

MB-23719: Add dynamic change of ephemeral_metadata_purge_age

Add ephemeral_metadata_purge_age to setFlushParam() so its value can
be changed dynamically via SET_PARAM.

Also add a requirement on bucket_type==ephemeral, as it's not valid
for other bucket types.

Change-Id: I0fecb5ced449c672dd95a4b196bdab9f6aaa7347
Reviewed-on: http://review.couchbase.org/76541
Reviewed-by: Manu Dhundi <manu@couchbase.com>
Tested-by: Build Bot <build@couchbase.com>