v2: Add a settable global DEFAULT_TIMEOUT

[coretl]

At the moment DEFAULT_TIMEOUT is hardcoded to 10s and used in a number of operations that should complete almost immediately. It would be useful to make this a global settable value for those times where something is being slow and you need just a bit more time.

This could be implemented in a few ways:

• Provide global setter and getter functions, and use the getter function wherever we use DEFAULT_TIMEOUT. Care is needed in functions that take DEFAULT_TIMEOUT as an argument, a sentinal value will be needed as the default, to be replaced by the value of the getter function when called
• Make DEFAULT_TIMEOUT not a float, but a custom class that evaluates to a float when float(DEFAULT_TIMEOUT) or num + DEFAULT_TIMEOUT is called. At least __float__() and __add__() would need to be overridden to support this.
• Other cleverness that I have not thought of

How much magic do we want here?

@tacaswell @prjemian @RAYemelyanova