Fix return values in PhutilKeyValueCacheStack
Summary:
`PhutilKeyValueCacheStack::setKeys()`, `PhutilKeyValueCacheStack::deleteKeys()`, `PhutilKeyValueCacheStack::destroyCache()` are all supposed to return `$this` like all other implementations of the methods in parent class `PhutilKeyValueCache` already do.
Thus add missing return statements.
Test Plan: Run static code analysis, don't get a complaint about a missing return value anymore.
Reviewers: O1 Blessed Committers, mainframe98
Reviewed By: O1 Blessed Committers, mainframe98
Subscribers: tobiaswiese, valerio.bozzolan, Matthew, Cigaryno
Differential Revision: https://we.phorge.it/D26093