API: Allow filtering keys in iiprop=extmetadata

Requesting iiprop=extmetadata can return a *lot* of meta data, when the
client may only be interested in certain keys. This adds a
"iiextmetadatafilter" parameter to allow the client to indicate which
keys it cares about.

Change-Id: Ie2185ec285e40359fffab4932bff8cab16071a53
Merge "API: Allow filtering keys in iiprop=extmetadata"