TRANS14 — Name clashes with current in PropagationContext (01)

Summary: I noticed that the latest draft of the OTS clears up the name clashes
with Coordinator and Terminator in TransIdentity, but hasn"t fixed the
clash with current in PropagationContext. At present I"m having to
rename this to currentTransaction, but this may not be "official".

There are a few other places in the current spec. which I think may need
slight modifications (assuming I haven"t missed the text that covers
them):

i) the description of Current doesn"t specify what default timeout value
will be given to begin if set_timeout has not been called prior to begin
for the particular client thread. This may be deliberate so as to allow
an implementation dependant choice, or it may simply be assumed to be
zero (no timeout). Either way, I think something should be mentioned in
the spec.