Write a concurrent ftp server.

The client must
- ask the client about the file name to transfer
- connect to server
- ask for transferring the file
- if the file exists, get the file from the socket and save it on disk
- else print error on the screen and exits

The server must:
- be concurrent
- the worker must 
  - look for the file and return OK is the file exists, or else NOT_EXIST
  - if the file exists read it from disk and send it on socket
  - close communication