Make stores of other services available via PreHandleContext

[netopyr]

• Add ReadableStores that allows to create any kind of store. (First version will be hard-coded.)
• Integrate ReadableStores in PreHandleContext and QueryContext.
• Make TransactionHandler.preHandle() expect only the context.

[nickpoorman]

@netopyr Will this allow one service to call another service's readable store in handle? For example, Freeze Service - freezeUpgrade needs to be able to call Network Service - getFileContent to get the upgrade file.

[netopyr]

This issues will build the base and integrate it with the pre-handle and query workflow only. We are just starting to re-design the handle workflow, therefore it is too early to do any modifications right now.