Previously, small allocations which were sampled as part of heap
profiling were rounded up to `SC_LARGE_MINCLASS`. This additional memory
usage becomes problematic when the page size is increased, as noted in #2358.
Small allocations are now rounded up to the nearest multiple of `PAGE`
instead, reducing the memory overhead by a factor of 4 in the most
extreme cases.
The previous approach managed the thread name in a separate buffer, which causes
races because the thread name update (triggered by new samples) can happen at
the same time as prof dumping (which reads the thread names) -- these two
operations are under separate locks to avoid blocking each other. Implemented
the thread name storage as part of the tdata struct, which resolves the lifetime
issue and also avoids internal alloc / dalloc during prof_sample.
The added hooks hooks.prof_sample and hooks.prof_sample_free are intended to
allow advanced users to track additional information, to enable new ways of
profiling on top of the jemalloc heap profile and sample features.
The sample hook is invoked after the allocation and backtracing, and forwards
the both the allocation and backtrace to the user hook; the sample_free hook
happens before the actual deallocation, and forwards only the ptr and usz to the
hook.
