Properly mark things that are deprecated as deprecated

[JeanChristopheMorinPerso]

Relates to #1552

Properly mark things that are deprecated as deprecated and add a new environment variable called REZ_LOG_DEPRECATION_WARNINGS to help users know when they are using deprecated things.

The new REZ_LOG_DEPRECATION_WARNINGS environment variable will hopefully allow our users to move away from deprecated things before they are removed from 3.0.0.

With this work, we can now deprecate pretty much anything, including config settings. Deprecation warnings for config settings will print the config file that generated the warning. It also works with config setting environment variables too.

(.venv) [jcmorin@arch01 rez]$ REZ_LOG_DEPRECATION_WARNINGS=1 REZ_WARN_OLD_COMMANDS=1 rez-search python
RezDeprecationWarning: config setting named 'warn_old_commands' (configured through the REZ_WARN_OLD_COMMANDS environment variable) is deprecated and will be removed in 3.0.0.
python-2.7.16
python-3.7.4

I tried to find a balance between being conservative and progressive:

• rez-1 config settings that are currently unused in the code base are deprecated and will be removed in 3.0.0.
• rez-1 Config settings that are still used in the code based are deprecated but are marked as "will be removed in the future".
• Two rez-1 config settings's default will be changed in 3.0.0. That is, disable_rez_1_compatibility will be enabled and rez_1_environment_variables will be disabled. Users can always override the defaults if they still need the old behavior.
• Things that were previously marked as deprecated now officially have a removal date (3.0.0). They aren't controversial things, so hopefully it's alright with everyone.

I added every deprecation to the change log in preparation of the release (and facilitate reviewing this PR) and I also wrote a transitioning guide for 3.0.0.

Hopefully, everything is clear and our users should find it straightforward and clear.

[JeanChristopheMorinPerso]

I initially wanted to deprecate it and remove it in 3.0.0, but I found out that this would be a little bit too disruptive for our oldest users. Same for rez_1_environment_variables. rez_1_environment_variables is even more painful because it's a lot harder to know if packages are actively using these old environment variables or not.