The lighter "struct mutex" is in a private header, so we can't
inline it in a public one, so just use pthread_mutex here.
By adjusting padding, the class size stays the same (72 bytes
on 32-bit, 96 bytes on 64-bit; confirmed via static_assert.)
A quick benchmark running "new/delete BMessage" in a loop on 4
threads at once goes from 30-36 seconds before this commit
to around 13-17 seconds, sometimes as low as 3 seconds, afterwards,
so clearly this is a significant improvement.
This also eliminates another BLocker allocated on application startup.
Augustin Cavalier
ae70142168