Changes

callstack.core: check monitor more often for callgraph analysis

This commit adds the possibility of canceling a very long callgraph
analysis. Without this, the callgraph analysis prevents Trace Compass
from exiting correctly and would not cancel properly.

Signed-off-by: Arnaud Fiorini <fiorini.arnaud@gmail.com>
Change-Id: Ib805a4aed35e9d4218b8f8b6a8ac7f245fff34c3