Let users add their own txid suffixes

The value of the X-Trans-Id-Extra header on the request (if any) will
now be appended to the transaction ID. This lets users put their own
information into transaction IDs.

For example, Glance folks upload images as large objects, so they'd
like to be able to tie together all the segment PUTs and the manifest
PUT with some operation ID in the logs. This would let them pass in
that operation ID as X-Trans-Id-Extra, and then when things went
wrong, it'd be much easier to find all the requests in Swift's logs.

Also, this aids debuggability when requests fail to receive
responses. If a user is sending in their own X-Trans-Id-Extra strings,
then that gives operators something to search for in the logs. The
normal txid won't work since that's in the response, but the client
didn't receive one.

Swift will only use the first 32 characters of X-Trans-Id-Extra so
that its log lines stay a manageable length. Also, it's URL-quoted so
that users cannot inject double quotes into X-Trans-Id-Extra and screw
up log parsers.

DocImpact

Change-Id: I3c51d0c5ac55697ac230001840da219e73a03157