Allow Longer keys upto 600 chars

Motivation: Earlier we were conservative about our key-size since
it is easier to increase but difficult to increase in backward
compatible fashion. Now with introduction of sub_namespace and
directly allowing upto 360char keys in LDK, we need support for
longer key lengths.
* We use varchar over text as it is sql standard
and can be indexed in all sql-implementations while limiting
key size.
* LDK can have keys of length up to 362(120*3+2), we use
600 as a safe limit with some buffer and considering applications
other than LDK.

Author: G8XSU

app/src/main/java/org/vss/impl/postgres/sql/v0_create_vss_db.sql

@@ -1,6 +1,6 @@
 CREATE TABLE vss_db (
     store_id character varying(120) NOT NULL CHECK (store_id <> ''),
-    key character varying(120) NOT NULL,
+    key character varying(600) NOT NULL,
     value bytea NULL,
     version bigint NOT NULL,
     PRIMARY KEY (store_id, key)