Queue up the tasks in limit-active-tasks.

Provide a DB based queueing system for tasks when
using the limit-active-tasks strategy.

The ordering is insertion-time based: first arrived
first served.

Tasks are assigned different queues when specifiying:
platform, team and tags.

Update package for Squirrel to include bugfix
for `FromSelect` not working properly.

FYI:
I was inspired by this talk: https://www.pgcon.org/2016/schedule/attachments/414_queues-pgcon-2016.pdf
for the design of the Postgres-based queue.

Signed-off-by: Alessandro Degano <[email protected]>

atc/atccmd/command.go

@@ -596,7 +596,8 @@ func (cmd *RunCommand) constructAPIMembers(
 	)
 
 	pool := worker.NewPool(workerProvider)
-	workerClient := worker.NewClient(pool, workerProvider)
+	taskQueue := db.NewTaskQueue(dbConn)
+	workerClient := worker.NewClient(pool, workerProvider, taskQueue)
 
 	credsManagers := cmd.CredentialManagers
 	dbPipelineFactory := db.NewPipelineFactory(dbConn, lockFactory)
@@ -763,7 +764,8 @@ func (cmd *RunCommand) constructBackendMembers(
 	)
 
 	pool := worker.NewPool(workerProvider)
-	workerClient := worker.NewClient(pool, workerProvider)
+	taskQueue := db.NewTaskQueue(dbConn)
+	workerClient := worker.NewClient(pool, workerProvider, taskQueue)
 
 	defaultLimits, err := cmd.parseDefaultLimits()
 	if err != nil {