storage: optimize multiple objects write (#1059)

* Fix tests

* storage: additional test cases

* db: add Pgx version of ExecuteInTx helper

* storage: optimize multiple objects write

Previously multiple objects write required 2 round trips
to DB per object: one to fetch object state from DB and second
to update object. Each query is indexed and fast, but
with latency to DB comparable to query execution time it adds
significant overhead.

This change optimizes multiple objects write in 2 steps:

1. Instead of reading DB state for each object and then deciding
   on possible write operation, perform write operation unconditionally
   with correct predicates (version and permissions) defined where
   applicable. That said write operation might not succeed if row doesn't
   match predicate. Write query is structured in such way, that
   final state of the row in the database is returned, regardless
   whether writeop successed or not. By inspecting returned row we can
   infer whether it was success, version conflict or permission error.

2. Now that each object is written to DB in a single query, there
   is no dependencies between queries and all of them can be blasted
   to DB in a batch without waiting for result of each. Whole batch
   continues to be executed in a single transaction, so outcome is
   the same, but batching negates latency penalty.