Merge branch 'v0.11' of github.com:cocaine/cocaine-core into v0.11
A more reasonable default value of grow threshold.

Default value queue_limit / pool_limit * concurrency may be more than queue_limit. It leads to the fact that available workers will not be started when they are needed. I have changed the value to queue_limit / pool_limit / 2. This value makes the runtime to run all available workers when the queue is half full.
Check if a pointer is not null before dereferencing it.

Channel may not have attached stream. In this case footprint equals 0.
Merge branch 'v0.11'
Merge branch 'master' of github.com:cocaine/cocaine-core