Enable hot standby PostgreSQL replicas.

Notes:
 - This still needs UI support from the Console
 - I've not tuned any GUCs for PostgreSQL to make this work better
 - Safekeeper has gotten a tweak in which WAL is sent and how: It now
sends zero-ed WAL data from the start of the timeline's first segment up to
the first byte of the timeline to be compatible with normal PostgreSQL
WAL streaming.
 - This includes the commits of #3714 

Fixes one part of https://github.com/neondatabase/neon/issues/769

Co-authored-by: Anastasia Lubennikova <anastasia@neon.tech>

docker-compose/compute_wrapper/var/db/postgres/specs/spec.json

@@ -28,11 +28,6 @@
                 "value": "replica",
                 "vartype": "enum"
             },
-            {
-                "name": "hot_standby",
-                "value": "on",
-                "vartype": "bool"
-            },
             {
                 "name": "wal_log_hints",
                 "value": "on",