Don't leak session upon writeErr
Don't leak session upon writeErr

Summary: We need to intiate a close if we have a writeError, else we never get around
to releasing the session.

Test Plan: run unit tests

Reviewed By: @alikhtarov

Differential Revision: D2278804
easyfmxu/mcrouter,glensc/mcrouter,synecdoche/mcrouter,is00hcw/mcrouter,facebook/mcrouter,facebook/mcrouter,nvaller/mcrouter,nvaller/mcrouter,easyfmxu/mcrouter,zhlong73/mcrouter,tempbottle/mcrouter,yqzhang/mcrouter,reddit/mcrouter,synecdoche/mcrouter,reddit/mcrouter,zhlong73/mcrouter,glensc/mcrouter,yqzhang/mcrouter,zhlong73/mcrouter,zhlong73/mcrouter,tempbottle/mcrouter,facebook/mcrouter,yqzhang/mcrouter,synecdoche/mcrouter,easyfmxu/mcrouter,facebook/mcrouter,reddit/mcrouter,is00hcw/mcrouter,nvaller/mcrouter,nvaller/mcrouter,easyfmxu/mcrouter,is00hcw/mcrouter,reddit/mcrouter,tempbottle/mcrouter,glensc/mcrouter,yqzhang/mcrouter,tempbottle/mcrouter,synecdoche/mcrouter,is00hcw/mcrouter,glensc/mcrouter