ART: Memory order updates to trace.{h,cc}

Adds comments and DCHECKS to clarify memory semantics.

Relaxes some atomic operations on Trace::cur_offset_ and makes its
initialization sequentially-consistent.

Adds GUARDED_BY annotations to fields only accessed with the
streaming_lock_.

Bug: 31023171
Test: art/test.py --host --stream -j32
Test: art/test.py --host --trace -j32
Test: art/test.py --target --stream -j4
Test: art/test.py --target --trace -j4
Change-Id: I31ad9a3861c7f258ed225207b3c2b39669ad8d36