Use constants for link metadata keys

Instead of using strings (which can be easy to
mistype and get wrong), provide a set of constants
that can be used to attach and use these keys in
flows and at compilation.

This also helps make it more clear what the keys
do and where they are used.

Change-Id: I5283b27617961136a4582bbcfce4617f05e8dd1d
Bump the deprecation version number

These will start to emit deprecation warning messages
in the next version/release (0.6) and not the current
version (0.5) so we should update the messaging to
reflect this.

Change-Id: I8823741847b42056690629fa12c612c8d1ef2cff
Use constants for retry automatically provided kwargs

Instead of using strings use module level constants for
the automatically provided keyword arguments to the retry
revert/execution functions. This makes it easier for users
of taskflow to associate these constants with the actual
keywords, without having to resort to using raw strings
directly.

Change-Id: I739a0ad69819c3ca0d10c98966012cf9ef1b86bd
Merge "Use constants for link metadata keys"
Merge "Use constants for retry automatically provided kwargs"
Merge "Bump the deprecation version number"