[FEA] Use `sentinel` namespace in all applicable map types

[PointKernel]

Is your feature request related to a problem? Please describe.
sentinel namespace has been added in #142 , which provides a wrapper around sentinel values. Currently, this namespace is only applied in cuco::static_map.

Describe the solution you'd like
Use sentinel namespace consistently across all applicable map types.

[m3g4d1v3r]

Hello PointKernel!
Is #163 a good approach for this issue? Maybe I'm forgetting something...