The agent was crashing with 'fatal error: concurrent map writes' when
handleCheckUpdatesCommand spawned a goroutine that called collectOnce
concurrently with the main collection loop. Both code paths access
a.prevContainerCPU without synchronization.
Added a.cpuMu mutex to protect all accesses to prevContainerCPU in:
- pruneStaleCPUSamples()
- collectContainer() delete operation
- calculateContainerCPUPercent()
Related to #1063