Take the trace out under the lock and ask for the quota outside it

[Thayorns]

print_trace! held the trace mutex while it asked for the quota. After 100 requests Fbe::Middleware::RateLimit stops serving /rate_limit from its cache and sends a real request, which passes through Fbe::Middleware::Trace, and that one locks the same mutex again on the same thread. So every run with 100 or more requests ended with ThreadError: deadlock; recursive locking, as #1224 shows.

Now print_trace! takes the whole trace out under the lock with slice!, releases the lock, and only then builds the message and calls rate_limit!. The trace is still emptied by the print, and the output is the same as before. Most of the diff is the body losing one level of indentation.

Two tests make over a hundred requests through a WebMock stub and print the trace, once and then twice past two quota refreshes. Both failed with the ThreadError before the change.

Closes #1224

[Thayorns]

@yegor256 take a look please, happy to clarify anything about this change.