Generalizing LinksUpdate to allow extensions to add arbitrary update handlers.

Generalizing LinksUpdate to allow extensions to add arbitrary update handlers.

This supercedes I6d03bf2a, using better names for the new classes and
incorporating the changes requested by Aaron.

This change introduces the base class SecondaryDataUpdate to be used for any
updates that need to be applied when a page is changed or deleted. Until now,
this was done by the LinksUpdate class for updates and WikiPage::doDeletionUpdates
upon deletion. This patch uses a list of SecondaryDataUpdates in both cases.

This allows extensions (e.g. via the ContentHandler facility, once that is in) to
easily specify what needs to be done when a page is updated or deleted in order to
keep any secondary data stores (such as link tables) in sync.

Note that limited transactional logic is also introduced, so SecondaryDataUpdate
can be implemented to only commit their changes if all updates were performed
sucessfully.

Patch Set 2: fixing some coding style issues mentioned by Nikerabbit.

Patch Set 4: some stuff I kept from the old LinksUpdate class needs cleanup,
             but might break extensions when changed. Marking as todo for now.

Patch Set 5: fixed misnamed member in LinksDeletionUpdate (thanks Aaron).

Change-Id: Ibe3e88fadd8c1d4063cf13bb6972f2a23569a73f