Thread pool

[OfekShochat]

changes:

• add thread pool for multi gpu use without multiple instances of client
• add quiet mode which suppresses serverParams print and Args print
• cons: adds 2 additional dependencies: "github.com/shettyh/threadpool" (threadpool) and "github.com/jaypipes/ghw" (gpu number) (so appveyor.yml was updated) and increase executable size from 7577kb to 8164kb

known issues:

• in task, if it encounters an error other thread will not stop. can be implemented with bool var that you check in every iteration of the for loop in main(), or using callable task (https://github.com/shettyh/threadpool) that also returns a future and use future.IsDone(). tho this is harder and probably messier.