Fix possible deadlock in the PostgreSQL specs

Force the lock_timeout and table_exists? queries to use the same
connection, otherwise they could use two different connections,
and the table_exists? query will hang until the database default
lock timeout.

This deadlock in the specs could only be hit if there were
3+ available connections in the connection pool before the
spec was running (1 for the lock, 1 for the lock_timeout,
1 for the table_exists). If 0-2 available connections were
in the pool, then 1 would be taken by the lock, and the 2nd
would be used for both the lock_timeout and table_exists?.

spec/adapters/postgres_spec.rb

@@ -80,12 +80,16 @@ def c.exec_prepared(*); super; nil end
       t = []
       @db[:test].lock('ACCESS EXCLUSIVE') do
         Thread.new do
-          @db.run "SET lock_timeout = 1"
-          t << @db.table_exists?(:test)
+          @db.synchronize do
+            @db.run "SET lock_timeout = 1"
+            t << @db.table_exists?(:test)
+          end
          end.join
         Thread.new do
-          @db.run "SET statement_timeout = 1"
-          t << @db.table_exists?(:test)
+          @db.synchronize do
+            @db.run "SET statement_timeout = 1"
+            t << @db.table_exists?(:test)
+          end
          end.join
       end
       t.must_equal [true, true]