perf(db) improve kong startup time by reusing luasocket on kong.init

Without this patch the Postgres strategy creates 7 non-pooled
luasocket connections to database on Kong.init(). This patch
reduces that to 3.

Without this patch the Cassandra strategy creates 9 non-pooled
luasocket connections to database on Kong.init(). This patch
reduces that to 1.

The difference is because Postgres needs to run initialization
SQL to setup the connection timezone and database schema on new
connections. And for Postgres `connect_migrations` is the same
as `connect` while with Cassandra it is not. It is still a tiny
improvement for Postgres as well.

kong/init.lua

@@ -289,6 +289,7 @@ function Kong.init()
   end
   --]]
 
+  assert(db:connect())
   assert(db.plugins:check_db_against_config(config.loaded_plugins))
 
   -- LEGACY
@@ -344,19 +345,19 @@ function Kong.init()
   sort_plugins_for_execution(config, db, loaded_plugins)
 
   local err
-  plugins_map_semaphore, err = semaphore.new()
+  plugins_map_semaphore, err = semaphore.new(1) -- 1 = treat this as a mutex
   if not plugins_map_semaphore then
     error("failed to create plugins map semaphore: " .. err)
   end
 
-  plugins_map_semaphore:post(1) -- one resource, treat this as a mutex
-
   local _, err = build_plugins_map(db, "init")
   if err then
     error("error building initial plugins map: ", err)
   end
 
   assert(runloop.build_router(db, "init"))
+
+  assert(db:close())
 end