Without this, the install_name of JAlienROOT is libJAlienROOT rather than
@rpath/libJAlienROOT, with the result things might break when SIP is
enabled.

1. Reduce reconnect log messages from Error to info
2. Replace cout printing with ROOT logging methods

Remove our implementation ::GetDataset() which used to return TDSet.
Let the TGridCollection::GetDataset() (super class) throw exception
because this method shouldn't be used anyway.

size

Flush() is void and we just continue even if there is an error

TNetXNGFile requires the file to be open to be able to run any size
checks

Let TJAlien decide if it should reconnect - remove recursive reconnect
from the connection manager.
Poll for connection status in IsConnected().
Use the right callback reason for the connection error event.

ROOT_CXX_FLAGS are added by include(${ROOT_USE_FILE})

This is required to get the Grid plugins working on
MacOS catalina and ROOT 6-18-xx.