Missing pidPageToMappingInfo keys

[christos68k]

Summary

Noticed a lot of errors with the current master:

ERRO[0300] failed to delete dummy prefix for PID 23385: batch delete: key does not exist 
ERRO[0300] failed to delete dummy prefix for PID 24074: batch delete: key does not exist 
ERRO[0300] failed to delete dummy prefix for PID 24148: batch delete: key does not exist 
ERRO[0300] failed to delete dummy prefix for PID 26289: batch delete: key does not exist 
ERRO[0300] failed to delete dummy prefix for PID 26931: batch delete: key does not exist 
ERRO[0300] failed to delete dummy prefix for PID 24314: batch delete: key does not exist 

I tracked the issue down to dd4c52b (PR: #325) so it seems that the refactoring we did in #325
did not maintain previous semantics / introduced a race.

For more context see this thread.

To reproduce, execute a workload that combines both CPU usage and spawns a lot of processes (e.g. build the agent)

[christos68k]

The easy fix is to partially revert #325 and restore the ProcessPIDExit logic to f094776. I'll dig some more to find out the root cause.