perf(find): resolve only the named design doc when use_index is given

[Crash--]

Problem

find() always calls getIndexes(db), which does an allDocs over the whole
_design/ range with include_docs: true — loading and massaging every query
design doc on every find(), even when the caller already named the index via
use_index. The planner then walks all of them only to keep the one that was named.

On a database with many mango indexes (and clients that always pass use_index), this is overhead proportional to the number of design docs, repeated per query. On React Native it serializes into a multi-second stall on a large replica.

Fix

When use_index is present, fetch just that one design doc with
db.get('_design/<ddoc>') and build the index list from it. If anything is unexpected
(missing ddoc, non-query language, malformed views, or the named index doesn't satisfy
the request), fall back to the original full getIndexes() path — so results and error
behaviour are unchanged (the existing unknown_error / no_usable_index cases still
raise the same errors).

Ported to async/await to match current master; adds a test asserting a use_index find
returns the same docs as the unindexed find.