Add coroutine for concurrency

From https://discuss.tryton.org/t/thread-less-long-polling/960
review53501002 adds a coroutine option to use gevent instead of multi-threaded server.

assigned to @ced

added trytond type::feature + 1 deleted label

review53501002 updated at https://codereview.tryton.org/53501002/#ps80001

New changeset f49e2e7fc2e6 by Cédric Krier in branch 'default':
Add coroutine concurrency option
https://hg.tryton.org/trytond/rev/f49e2e7fc2e6

added 1 deleted label and removed 1 deleted label

closed

New changeset 5f6dab1a4cba by Cédric Krier in branch 'default':
Add coroutine concurrency option
https://hg.tryton.org/tryton-env/rev/5f6dab1a4cba