[connection] remove the `mode` attribute It is now unused. While removing mode, we also drop some sqlserver-specific reconnection logic snippets. These had several downsides: * untested * partial coverage * done at the wrong level Related to #2919309.

[connection] remove ensure_cnx_set context manager uses It has been deprecated in a previous changeset and is now a no-op. Related to #2919309.

[server] remove cnxset tracking, it is now unneeded Indeed, all cnxsets will now be acquired using the following API:: with session.new_cnx() as cnx: cnx.execute(...) # do stuff cnx.commit() These well-scoped blocks are the only place a cnxset can be acquired. The old use-case for un-delimited cnxsets (pyro/zmqpickle protocols) have been removed. The "mode" of connection objects becomes "write" forever (it will be removed in a couple of changesets). Related to #2919309.