Improve extension engine compatibility

[joaomoreno]

Currently, an extension is deemed outdated when there's a newer version published in the Marketplace. Installing an extension requires O(published_versions) request roundtrips: starting from the latest version, the engine field is checked for compatibility with the current Code, and it goes on backwards from there.

We should:

• Publish extensions with the compatibility value as a property
• Use that property to figure out which version to install
• Use that property to correctly figure out whether the extension is outdated

VSIX manifest format:

<Properties>
      <Property Id="abc" Value="def" />
      <Property Id="ghi" Value="jkl" />
</Properties>

Response:

"version": "0.1.15",
              "flags": "validated",
              "lastUpdated": "2016-09-06T04:41:03.74Z",
              "properties": [
                {
                  "key": "abc",
                  "value": "def"
                },
                {
                  "key": "ghi",
                  "value": "jkl"
                }
]

[joaomoreno]

@rebornix just hit the bug that this will fix: an extension which has a new version published, which isn't compatible with the user's version of Code, always appears out of date in the product.

[daviwil]

I wonder if this explains what @gerane was seeing in issue #12281?

[joaomoreno]

Probably not, since this is about update notifications.

[gerane]

Not sure. My issue is that code is not able to connect some of my installed extensions to to the ones in the marketplace any longer. I don't get update notifications for them, but not sure if those mechanisms are linked in anyway.

Also, I'm pretty sure this issue was introduced a while ago, but it's not an easy one to catch.