Changed the SocketManager to pass "Sockets" by value.
Changed the SocketManager to pass "Sockets" by value.

`Sockets` is already a reference-counted `shared_ptr` under the covers.
By passing around `Sockets` by value, we avoid potentially deleting
a reference while the same reference is in use by another function.

Review: https://reviews.apache.org/r/49404/

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