Prefetching: Introduce store commands abstractions to be used by tasks.

components/offline_pages/core/prefetch/README.md

-# Prefetching Offline Pages: development guidelines
+# Prefetching Offline Pages
 
-* Implementations that are injected dependencies should always provide
-  lightweight construction and postpone heavier initialization (i.e. DB
+## Architecture overview
+
+### PrefetchService
+
+Is the ownership holder for the main components of the prefetching system and
+controls their lifetime.
+
+### PrefetchDispatcher
+
+Manages the prefetching pipeline tasks. It receives signals from external
+clients and creates the appropriate tasks to execute them. It _might_ at some
+point execute advanced task management operations like canceling queued tasks or
+changing their order of execution.
+
+### \*Task(s) (i.e. AddUniqueUrlsTask)
+
+They are the main wrapper of pipeline steps and interact with different 
+abstracted components (Downloads, persistent store, GCM, etc) to execute them.
+They implement TaskQueue's Task API so that they can be exclusively executed.
+
+## Development guidelines
+
+* Implementations that are injected dependencies during service creation should
+  have lightweight construction and postpone heavier initialization (i.e. DB
   connection) to a later moment. Lazy initialization upon first actual usage is
   recommended.