[JENKINS-30139] Switch from a big fat lock to a concurrent collection

[JENKINS-30374] Update obsolete help message

Remove obsolete reference to "location you configured above" since the location is now determined automatically rather than explicitly configured.

Use slf4j adapters for logging.

 By means of this *all* logging should go to JUL.  By defining slf4j-api as compile
 scope we mark it as the one to be used, while by declaring it optional we make
 sure it is not packaged in the resulting HPI.

Document allBuilds for Job remote access API

- By introducing adapters for log4j and commons-logging and
- including the original log4j and commons-logging dependencies as provided
all logging should be redirected to slf4j and finally java.util.logging.

[JENKINS-28440] Additional fixes for #1715

Upgrade HtmlUnit to v2.18 (from forked version)

* master:
  commons-compress -> 1.10
  Noting #1788
  Remove hardcode in tar test
  [FIXED JENKINS-10629] Unbroke stream with new tar implementation.
  Revert "Revert "FIXED JENKINS-10629] - Enable BigNumber mode to support archiving of files with size >8Gb""
  Revert "Revert "[JENKINS-10629] - Migrate the Tar archives handling code to commons-compress""
  Fix at-since from PR #1788
  Merge PR #1788: Make plugin manager pluggable

Causes bytes lost and truncated tar archive.
TarBuffer not used in TarArchiveOutputStream.