Add mutexes to client examples to avoid race conditions

[dawidpieper]

When streaming a lot of data or opening a lot of streams it is possible that HTTP2::Client#on(:bytes) will call blocks before completion of the previous one. I suggest to add mutexes to examples to present how to avoid this situation resulting in OpenSSL errors.

[coveralls]

Coverage remained the same at 97.313% when pulling 766c2351539b7117a1a701b3da391e735dc8ce4b on dawidpieper:examples_add_client_mutexes into 8ec79e8982cb65fb9740ed8ca6e1950774a5a533 on igrigorik:master.