Make rpc/lmp thread safe

[abertschi]

When using locks be careful:

• client requests RAM cap which triggers pagefault which requests RAM cap to map a new frame.

[Liblor]

we might have a concurrency issue at https://github.com/Liblor/advanced_operating_systems_2020/tree/morecore-static-init-debug

a vaddr_region is marked as mapped/allocated even though it is not really mapped