tsan: reduce history size in Go mode

The ContainsSameAccess optimization substantially reduces pressure
on trace by eliminating duplicate accesses. So now we can reduce
default trace size to reduce per-goroutine memory consumption.
Current default size is 64K events, new -- 32K events.
In either case user can change it with GORACE env var.

Reduces per-goroutine memory consumption from 356K to 226K.

llvm-svn: 229117