Add ability to freeze list of iotile plugins

[timburke]

When running on an SBC with an SD card, enumerating the filesystem inside pkg_resources to find iotile plugins is very slow, adding 5-10 seconds to the start up time of any iotile command line script. It's not possible to actually speed up pkg_resources but in situations where we know that the user is not installing / removing plugins (i.e. when the file system is read only), we can prescan all installed plugins and produce an entry_points.txt file similar to the component registry that contains all of the entry points.

If this file exists it can be consulted before pkg_resources is imported to speed up application load time:

See pkg_resources slow behavior, known bug, here:
pypa/setuptools#926

[timburke]

This will require changes across coretools:

• A new class in iotile.core.utilities called EntryPointManager that checks for entry_points.txt and loads it otherwise it loads pkg_resources. It should have an iter_entry_points API as a drop in replacement for pkg_resources usage.
• Port all Coretools packages to use EntryPointManager rather than directly using pkg_resources.

For this to be useful it has to be an all or nothing thing. If anybody still imports pkg_resources the slow performance will return.

[timburke]

It seems like a lot of larger projects have the same issue with pkg_resources.iter_entry_points and are settling on the alternative entrypoints package: marcelotduarte/cx_Freeze#216 (comment).

[timburke]

This basic ability was added in PR #645. You can now iotile registry freeze and iotile registry unfreeze, however there are a few remaining items:

1. This only affects parts of CoreTools that have been ported over to use the centralized extension system Refactor the rest of CoreTools to use centralized extension system #624. Any part of CoreTools still using pkg_resources directly will not pick up the frozen plugins.

2. There needs to be a clear logging message on program start saying that frozen plugins are being used so that users don't get confused and wonder why their newly pip installed packages aren't being detected.