Fix blocking operations from missing new lists

Behrad Zari discovered [1] and Josiah reported [2]: if you block
and wait for a list to exist, but the list creates from
a non-push command, the blocked client never gets notified.

This commit adds notification of blocked clients into
the DB layer and away from individual commands.

Lists can be created by [LR]PUSH, SORT..STORE, RENAME, MOVE,
and RESTORE.  Previously, blocked client notifications were
only triggered by [LR]PUSH.  Your client would never get
notified if a list were created by SORT..STORE or RENAME or
a RESTORE, etc.

Blocked client notification now happens in one unified place:
  - dbAdd() triggers notification when adding a list to the DB

Two new tests are added that fail prior to this commit.

All test pass.

Fixes #1668

[1]: https://groups.google.com/forum/#!topic/redis-db/k4oWfMkN1NU
[2]: #1668

tests/unit/type/list.tcl

@@ -408,6 +408,29 @@ start_server {
       $rd read
     } {}
 
+    test "BLPOP when new key is moved into place" {
+        set rd [redis_deferring_client]
+
+        $rd blpop foo 5
+        r lpush bob abc def hij
+        r rename bob foo
+        $rd read
+    } {foo hij}
+
+    test "BLPOP when result key is created by SORT..STORE" {
+        set rd [redis_deferring_client]
+
+        # zero out list from previous test without explicit delete
+        r lpop foo
+        r lpop foo
+        r lpop foo
+
+        $rd blpop foo 5
+        r lpush notfoo hello hola aguacate konichiwa zanzibar
+        r sort notfoo ALPHA store foo
+        $rd read
+    } {foo aguacate}
+
     foreach {pop} {BLPOP BRPOP} {
         test "$pop: with single empty list argument" {
             set rd [redis_deferring_client]