Remove straggler references to advisory locks for uniqueness

Remove a few straggler references saying that advisory locks provide
uniqueness. As of #28, this is no longer true and a unique index is used
instead.

Author: brandur

docs/README.md

@@ -84,16 +84,6 @@ insert_res.job
 insert_res.unique_skipped_as_duplicated
 ```
 
-### Custom advisory lock prefix
-
-Unique job insertion takes a Postgres advisory lock to make sure that its uniqueness check still works even if two conflicting insert operations are occurring in parallel. Postgres advisory locks share a global 64-bit namespace, which is a large enough space that it's unlikely for two advisory locks to ever conflict, but to _guarantee_ that River's advisory locks never interfere with an application's, River can be configured with a 32-bit advisory lock prefix which it will use for all its locks:
-
-```ruby
-client = River::Client.new(mock_driver)
-```
-
-Doing so has the downside of leaving only 32 bits for River's locks (64 bits total - 32-bit prefix), making them somewhat more likely to conflict with each other.
-
 ## Inserting jobs in bulk
 
 Use `#insert_many` to bulk insert jobs as a single operation for improved efficiency:

lib/insert_opts.rb

@@ -72,12 +72,6 @@ def initialize(
   # given job kind, a single instance is allowed for each combination of args
   # and queues. If either args or queue is changed on a new job, it's allowed to
   # be inserted as a new job.
-  #
-  # Uniquenes is checked at insert time by taking a Postgres advisory lock,
-  # doing a look up for an equivalent row, and inserting only if none was found.
-  # There's no database-level mechanism that guarantees jobs stay unique, so if
-  # an equivalent row is inserted out of band (or batch inserted, where a unique
-  # check doesn't occur), it's conceivable that duplicates could coexist.
   class UniqueOpts
     # Indicates that uniqueness should be enforced for any specific instance of
     # encoded args for a job.