Use fully qualified modules in connection proxy

An instance of this class is stored in memory in a class instance
variable. When this class is stale, it causes code-reloading issues
when `Gitlab::Database::LoadBalancing::ConnectionProxy.const_missing` is
triggered.

This is a workaround so that `Object.const_missing` is triggered instead

ee/lib/gitlab/database/load_balancing/connection_proxy.rb

@@ -64,7 +64,7 @@ module Gitlab
         #
         # name - The name of the method to call on a connection object.
         def read_using_load_balancer(name, args, &block)
-          method = Session.current.use_primary? ? :read_write : :read
+          method = ::Gitlab::Database::LoadBalancing::Session.current.use_primary? ? :read_write : :read
 
           @load_balancer.send(method) do |connection|
             connection.send(name, *args, &block)
@@ -81,7 +81,7 @@ module Gitlab
             # Sticking has to be enabled before calling the method. Not doing so
             # could lead to methods called in a block still being performed on a
             # secondary instead of on a primary (when necessary).
-            Session.current.write! if sticky
+            ::Gitlab::Database::LoadBalancing::Session.current.write! if sticky
 
             connection.send(name, *args, &block)
           end