Wait until all Gitaly sessions are executed

If they haven't been executed within a timeout, we unblock the execution.

Related issue: #563 (closed)

We have an issue when a lot of git clones operations get canceled. It was assumed that some clients close the connection just after all the data has been received from Git server. If there was a network delay and gitlab-sshd hadn't managed to gracefully close the connection, context gets canceled and Gitaly cancels the execution and returns the error.

Let's wait for a perion to allow Gitaly to gracefully complete the operation.