pkg/lock: make time of internalRWMutex private
pkg/lock: make time of internalRWMutex private

This avoids developers on using methods from time.Time when using the
lock.Mutex structure.

Signed-off-by: André Martins <bc9800b9d52a24cce72a73dd528afed53f10e5fc@cilium.io>

cilium/cilium,tgraf/cilium,tgraf/cilium,scanf/cilium,tgraf/cilium,michi-covalent/cilium,michi-covalent/cilium,michi-covalent/cilium,tklauser/cilium,scanf/cilium,cilium-team/cilium,scanf/cilium,tgraf/cilium,scanf/cilium,tklauser/cilium,cilium/cilium,tklauser/cilium,cilium/cilium,cilium/cilium,cilium/cilium,michi-covalent/cilium,scanf/cilium,tgraf/cilium,michi-covalent/cilium,scanf/cilium,tklauser/cilium,cilium-team/cilium,tgraf/cilium,scanf/cilium,tklauser/cilium