Fix doFinally rx extension

[solomidSF]

Expected behavior

Underlying peripheral queue should execute all operations in sequential manner.

The issue

Peripheral queue is not executing operations sequentially when there are more than 2 operations enqueued.
Receiving side would get interleaving chunks for different incoming packets.

Reason

public func queue<O: GattOperation>(operation: O) -> Single<O.Element> returns sequence with injected side-effects utilizing internal state to manage/run operation queue in case there are more operations to be executed. doFinally rx extension was added to enqueue next operation when the current one finishes, however it's not implemented correctly.
Rx lifecycle usually looking like this:

==...==completed==disposed=|
==...==error==disposed=|
Please note: subscription is disposed immediately after completed/error event occurs.

doFinally is called twice per single operation (e.g. completed, disposed) and due to side-effects utilizing internal state it skips freshly enqueued operation (which will still complete!) and moves to the next one.

Example:

• Schedule 3 write operations
• Write1 completes
• doFinally is executed for Write1 because of the completed event so Write #2 is enqueued
• doFinally is executed again for Write1 because subscription will be disposed so Write #3 will be enqueued
• We are now sending packets both for Write2 and Write3 simultaneously

Added test to cover this behavior. Revert my change to doFinally to see test failing with previous logic: receiving side would get "Packet 1 containing dummy dataPackeP3t 2 containing slightly more dummy data than first packet" instead of "Packet 1 containing dummy dataPacket 2 containing slightly more dummy data than first packetP3"

Fix

Added variable to guard against multiple invocations.

[CLAassistant]

All committers have signed the CLA.

[jpsoultanis]

Thanks for submitting a fix for this issue :)