TRANS14 — Name clashes with current in PropagationContext (02)

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".
ii) I"ve been assuming that the resume operation on Current simply
"overwrites" the current transaction in favour of any that may be
running. So if the new transaction terminates, the client thread becomes
associated with the null context (assuming the transaction was not
nested.) However, it"s been pointed out to me that the spec. only says
the new transaction is used "in place of any previous transaction", and
that this could mean the old transaction is associated with the client
thread once the new transaction terminates. Whether or not this was
meant to be another implementation specific choice I think we need to
say something more in the description of resume.