Client retained outgoing packets semantics

[markpapadakis]

We should probably make sure that the aggregate size in bytes of all retained packets (for requests pending a response) does not exceed some value so that we won't run out of memory; if it reaches that threshold, new requests should be dropped immediately(and return 0 to the callee).

[markpapadakis]

Not sure this makes any sense now that TANK2 manages resources differently, but I am keeping this issue here just in case.