AsyncUDPSocket: abstract do_read function
AsyncUDPSocket: abstract do_read function

This basically implements read handling, without catching all
exceptions. When using the socket in synchronous mode, it's useful to
avoid losing exception data (which, in an async daemon, can only be
logged)

Signed-off-by: Guido Trotter <f62ca4d5358bdaeef0e50e57f9f7e231de63727e@google.com>
Reviewed-by: Michael Hanselmann <68b67885ff74281219b4a4be33cc20fd45078d7c@google.com>

yiannist/ganeti,bitemyapp/ganeti,apyrgio/ganeti,grnet/snf-ganeti,andir/ganeti,bitemyapp/ganeti,yiannist/ganeti,apyrgio/ganeti,onponomarev/ganeti,ganeti-github-testing/ganeti-test-1,ganeti/ganeti,dimara/ganeti,leshchevds/ganeti,onponomarev/ganeti,apyrgio/snf-ganeti,mbakke/ganeti,ganeti-github-testing/ganeti-test-1,andir/ganeti,ganeti/ganeti,mbakke/ganeti,dimara/ganeti,grnet/snf-ganeti,mbakke/ganeti,leshchevds/ganeti,yiannist/ganeti,andir/ganeti,apyrgio/snf-ganeti,ganeti/ganeti,leshchevds/ganeti