[FIX JENKINS-43228] Consider time zone for cron validation

* Clean up usages of IOUtils.closeQuietly and related calls predating Java 7.

* Review comments from @oleg-nenashev.

* Noticed another place where try-with-resources would protect against a potential leak.

[JENKINS-42852] - Jenkins Configuration Save Option

This reverts commit 5caee586.
In fact ssh-cli-auth contains UserPropertyImpl which is used by the sshd module.

* [FIXED JENKINS-40848] Introduce status indicator for skipped download job

* [JENKINS-40848] Stick to existing code-flow as that is proved to work

* [JENKINS-42645] Case insensitive search by default

* [JENKINS-42960] Search in FixedSet more locale friendly

String.equalsIgnoreCase is safer than toLowerCase when non English
locales are used.

* [JENKINS-42645] Review remarks

Verifying that PlainCLIProtocol ignores unrecognized opcodes, so long as it has received well-formed frames.

@stephenc requested documentation of the deprecated FullDuplexHttpStream constructor, and as it turns out it did not work.

Update node, npm, download-maven-plugin, & frontend-maven-plugin

Something¹ seems to interrupt the HTTP duplex service thread,
and Jetty apparently deals poorly with interruptions.
So introduce an extra sleep to soak up the interrupt.

¹Who? http://stackoverflow.com/questions/2126997/who-is-calling-the-java-thread-interrupt-method-if-im-not#comment40865453_2127397

Jetty 9.4.3.v20170317 refuses to set an empty HTTP header, leading to cryptic client errors since Hudson-Duplex is missing.

[JENKINS-27624][JENKINS-42709] Bump Jenkins baseline to Java 8 minimum