Use special context keys for automatic context values

[jlowin]

(inspired by work in #18)

Currently, when a flow adds itself to a context, it does so under the flow key (in other words, it becomes available at prefect.context.flow). This has the potential to override a possibly common user-supplied context value. Instead, any automatically-set context value (including flows, as described here, and all settings used by FlowRunner and TaskRunners, like parameters) should be put under a key, perhaps __prefect__. This way they won't shadow user variables.

[jlowin]

➤ Jeremiah Lowin commented:

After talking with @cicdw, going to change the key to __key instead of key