Resolve "Index namespaces.traversal_ids"

What does this MR do?

Index the namespaces.traversal_ids for fast lookup.

The index takes 34 seconds to be created in postgres.ai and it is pretty small (as expected as all traversal_ids are empty)

 Schema |               Name                | Type  |  Owner  |   Table    | Size  | Description   
--------+-----------------------------------+-------+---------+------------+-------+-------------  
 public | index_namespaces_on_traversal_ids | index | joe_122 | namespaces | 11 MB |   

DB Migrations

$ bundle exec rake db:migrate
== 20210323064751 AddNamespaceTraversalIdsIndex: migrating ====================
-- transaction_open?()
   -> 0.0000s
-- index_exists?(:namespaces, :traversal_ids, {:using=>:gin, :name=>"index_namespaces_on_traversal_ids", :algorithm=>:concurrently})
   -> 0.0068s
-- execute("SET statement_timeout TO 0")
   -> 0.0004s
-- add_index(:namespaces, :traversal_ids, {:using=>:gin, :name=>"index_namespaces_on_traversal_ids", :algorithm=>:concurrently})
   -> 0.0031s
-- execute("RESET ALL")
   -> 0.0004s
== 20210323064751 AddNamespaceTraversalIdsIndex: migrated (0.0115s) ===========

$ bundle exec rake db:rollback
== 20210323064751 AddNamespaceTraversalIdsIndex: reverting ====================
-- transaction_open?()
   -> 0.0000s
-- indexes(:namespaces)
   -> 0.0066s
-- execute("SET statement_timeout TO 0")
   -> 0.0005s
-- remove_index(:namespaces, {:algorithm=>:concurrently, :name=>"index_namespaces_on_traversal_ids"})
   -> 0.0029s
-- execute("RESET ALL")
   -> 0.0005s
== 20210323064751 AddNamespaceTraversalIdsIndex: reverted (0.0113s) ===========

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Related to #325636 (closed)