DPL: improve reliability of --driver-client-backend ws://

[ktf]

• Flush messages after handshake
• Fix handling of splitted frames
• Move to ControlServiceHelpers::processCommand()

[ktf]

@davidrohr this should improve things using --driver-client-backend ws://.

[davidrohr]

@ktf : I have tested this fix:

• Full system test still hanging (https://alice.its.cern.ch/jira/browse/O2-2014), both with and without --driver-client-backend ws://
• The error reported in private communication ([3405:MIDDigitsReader]: [18:42:54][WARN] Unable to communicate with driver because client does not exist) testing https://alice.its.cern.ch/jira/browse/O2-1887 with --driver-client-backend ws:// is fixed.
• Error reported in private communication (double free or corruption (!prev)) for https://alice.its.cern.ch/jira/browse/O2-1898 with --driver-client-backend ws:// is fixed.
• No improvement in the general behavior for O2-1887, O2-1898, O2-1900, etc. i.e.:
  • --driver-client-backend ws:// seems to work now, but it doesn't solve any of the issues reported in these tickets.
  • The full system test is still broken after Add WS DPL DriverClient. #5284

[ktf]

Merging as it improves things.