MINOR: activity: use dynamic allocation for "show profiling" entries

MINOR: activity: use dynamic allocation for "show profiling" entries

Historically, the data manipulated by "show profiling" were copied
onto the stack for sorting and aggregating, but not only this limits
the number of entries we can keep, but it also has an impact on CPU
usage (having to redo the whole copy+sort upon each resume) and the
output accuracy (if sorting changes lines, resume may happen from an
incorrect one).

Instead, let's dynamically allocate the work buffer and place it into
the service context. We only allocate it immediately before needing it
and release it immediately afterwards so that it doesn't stay long. It
also requires a release handler to release those allocates by interrupted
dumps, but that's all. The overall result is now much cleaner, more
accurate, faster and safer.

This patch may be backported to older LTS releases.