Implement conistent handling of timeouts.

[elprans]

The timeout logic is currently a bit of a mess. This commit attempts to
tidy things up.

Most importantly, the timeout budget is now applied consistently to the
entire call, whereas previously multiple consecutive operations used
the same timeout value, making it possible for the overall run time to
exceed the timeout.

Secondly, tighten the validation for timeouts: booleans are not
accepted, and neither any value that cannot be converted to float.

[1st1]

Rewrite this block to:

try: if isinstance(command_timeout, bool): raise ValueError command_timeout = float(command_timeout) if command_timeout < 0: raise ValueError except ValueError: raise ValueError( 'invalid command_timeout value: ' 'expected non-negative float (got {!r})'.format( command_timeout)) from None

[1st1]

if timeout is not None: try: if type(timeout) is bool: raise ValueError timeout = float(timeout) except ValueError: raise ValueError( 'invalid timeout value: expected non-negative float ' '(got {!r})'.format(timeout)) from None

[1st1]

Even though I suggested to split _do_execute in two methods myself, I now think we'd be better off to merge them in one :) The actual timeouts arithmetic happens just in two places, so we can just add two if statements to _do_execute.