Align with proposed standards for MP Config property names defined by MicroProfile specs

[njr-11]

Under eclipse/microprofile#100, new requirements/guidance is provided on how MicroProfile specs should name MP Config properties that they define. These properties must start with mp.[spec-name].

Under eclipse/microprofile-config#420, there is a request to ban the / character in property names.

This issue is opened for MicroProfile Context Propagation to pre-emptively align our spec-defined MP Config property names with both of these such that they will remain valid regardless of whether both/just one/neither of these proposals are adopted.

Property names will be:
mp.context.ManagedExecutor.cleared
mp.context.ManagedExecutor.maxAsync
mp.context.ManagedExecutor.maxQueued
mp.context.ManagedExecutor.propagated
mp.context.ThreadContext.cleared
mp.context.ThreadContext.propagated
mp.context.ThreadContext.unchanged