don't obtain stack traces on rbf.Tx creation

We thought stack traces were mildly expensive. We were very wrong.
Due to a complicated issue in the Go runtime, simultaneous requests
for stack traces end up contending on a lock even when they're not
actually contending on any resources. I've filed a ticket in the
Go issue tracker for this:

	golang/go#56400

In the mean time: Under some workloads, we were seeing 85% of all
CPU time go into the stack backtraces, of which 81% went into the
contention on those locks. But even if you take away the contention,
that leaves us with 4/19 of all CPU time in our code going into
building those stack backtraces. That's a lot of overhead for a
feature we virtually never use.

We might consider adding a backtrace functionality here, possibly
using `runtime.Callers` which is much lower overhead, and allows us
to generate a backtrace on demand (no argument values available,
but then, we never read those because they're unformatted hex
values), but I don't think it's actually very informative to know
what the stack traces were of the Tx; they don't necessarily reflect
the current state of any ongoing use of the Tx, so we can't necessarily
correlate them to goroutine stack dumps, and so on.

Author: seebs