[2a] Fetch and tokenize books in a host BookService #435

Part of Phase 2 (#425) of the data layer plan.

BookService in the host with a temporary getTokenizedBook, removed in 5d; it tokenizes only once the project's language tag has loaded, and caches per book and tag (#421); the WebView stops fetching and tokenizing USJ.

Done when: tokens match the WebView tokenizer's output on the #407 datasets, and opening a book tokenizes it once.

Depends on: #407 (done).

From the plan

API