creating many short sessions in parallel leads to errors

Test:
1. create several treads (e.g. 20)
2. every tread opens connections to Maxscale router (tested with RWSplit, but probably same applies for all), does short query ('select 1'), closes session in the loop

Expected result:
sessions are opened, queries are executed, sessions are closed normally

Actual result:

after a while query fails with:

Error: can't execute SQL-query: select 1
Authentication with backend failed. Session will be closed.

session can not b created any more: "failed to create new session"

Maxscale log:

2016-03-11 13:32:56 error : Invalid authentication message from backend. Error code: 1129, Msg : Host 'maxscale' is blocked because of many connection errors; unblock with 'mysqladmin flush-hosts'
2016-03-11 13:32:56 error : Server server1 has been put into maintenance mode due to the server blocking connections from MaxScale. Run 'mysqladmin -h 192.168.121.76 -P 3306 flush-hosts' on this server before taking this server out of maintenance mode.
2016-03-11 13:32:56 error : Could not find master among the backend servers. Previous master's state : NO STATUS
2016-03-11 13:32:56 error : Routing the query failed. Session will be closed.
2016-03-11 13:32:56 error : Could not find master among the backend servers. Previous master's state : NO STATUS
2016-03-11 13:32:56 error : Routing the query failed. Session will be closed.
2016-03-11 13:32:56 error : Could not find master among the backend servers. Previous master's state : NO STATUS
2016-03-11 13:32:56 error : Routing the query failed. Session will be closed.
2016-03-11 13:32:56 error : Could not find master among the backend servers. Previous master's state : NO STATUS
2016-03-11 13:32:56 error : Routing the query failed. Session will be closed.
2016-03-11 13:32:56 error : Invalid authentication message from backend. Error code: 1129, Msg : Host 'maxscale' is blocked because of many connection errors; unblock with 'mysqladmin flush-hosts'

If during every session some session command is executed (e.g. 'USE test') - no any failure.

Test cases: open_close_connections'
(currently contains 'USE test')