(cherry picked from commit df9c92e4)

(cherry picked from commit 1ac77750)

Conflicts:
	core/src/main/java/hudson/model/DownloadService.java
	core/src/main/java/hudson/model/UpdateSite.java

Since sorting of plugins in the update manager is based on displayName
this name should not have a common prefix otherwise it's hard to find
a given plugin name.

[FIXED JENKINS-19739] When installing a plugin and the needed dependencies have compatibility issues, warn the user

[FIXED JENKINS-19736] Installing a plugin with optional dependencies doesn't upgrade the optional dependencies when needed

Due to a confusion between 0 and -1. Easier and clearer to use 0 as the “uninitialized” value.

I had to tweak UpdateSite code because JSONObject.fromObject(String)
became more strict and no longer allows extra garbage after the closing
'}'

Removing old static field, which was not friendly to tests, especially multithreaded runners.
Cleaned up existing tests to use updateDirectly rather than POSTing raw JSON.

Correctly support direct updates of html update sites and other issues pointed out by jglick in https://github.com/jenkinsci/jenkins/pull/692

[FIXED JENKINS-15889] Saving the update center list after the metadata has been fetched results in the metadata being persisted twice

[FIX JENKINS-10569] When installing plugins with overlapping dependencies, Jenkins downloads the duplicate plugins multiple times

exposed data about installed plugins, installable plugins, update center jobs, etc., to the REST API

If we fail to obtain the update center metadata, don't keep retrying
every 15 seconds, but use exponential time out to avoid overloading
our server in case of a problem.

In this way we can keep both new and old happy.

Somebody was forgetting to close the stream and the resulting signature was therefore at the mercy of the buffer's flush in order to determine whether the signature was correct or not.

3a38a67c that causes update center to
fail to accept updates.

HTTPS