[XRay] Update FDR log reader to be aware of buffer sizes per thread.

[XRay] Update FDR log reader to be aware of buffer sizes per thread.

Summary:
It is problematic for this reader that it expects to read data from
several threads, but the header or message format does not define
framing. Since the buffers are reused, we can't rely on skipping
zeroed out data as a synchronization method either.

There is an argument that this is not version compatible with the format
the reader expected previously. I argue that since the writer wrote garbage
past the end of buffer record, there is no currently working reader to
compromise.

The corresponding writer change is posted to D31384.

Reviewers: dberris, pelikan

Reviewed By: dberris

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D31385

llvm-svn: 298983