Added option to not flush stream buffers on every message send

[tigerquoll]

Provides feature requested in #49

Adds a property to TcpSyslogMessageSender, "SocketFlush", with getters and setters:
Accepted values:

• SocketFlush.onSend - Default value - continues existing behaviour of flushing at every send, to ensure that all data is passed to the syslog server.
• SocketFlush.onClose -New behaviour to only flush stream buffers on syslog client close. This is higher performance but risks a small amount dataloss if the syslog server reboots.

[tigerquoll]

@cyrille-leclerc, could I ping you for a review please?

[cyrille-leclerc]

@tigerquoll I'm sorry but I lost access to this repository 2 years ago when I left CloudBees.
@bwalding can someone from CloudBees help clarify the future of this library?

[lemeurherve]

@cyrille-leclerc @tigerquoll FYI, a thread has been started on the jenkinsci-dev mailing list:
[Proposal] Transfer of CloudBees-community/syslog-java-client to jenkinsci Github org

[basil]

Hi @cyrille-leclerc, gentle friendly ping: if you would like access to this repository, please respond in the mailing list thread. If not, no problem! If nobody claims this repository within two weeks, then I will adopt it as interim maintainer to complete the transfer to the jenkinsci GitHub organization, but I cannot promise timely review, merge, or release of the PR backlog.

[cyrille-leclerc]

I'm back from PTO. I replied to the mailing list thread to support this proposal https://groups.google.com/g/jenkinsci-dev/c/ZmNbkgCadUY/m/NvvT4axhAAAJ

[tigerquoll]

Hi @cyrille-leclerc, are you still able to review?

[cyrille-leclerc]

@tigerquoll I'm sorry but I'm no longer an active maintainer of this library and I don't have much time at the moment.
@basil @lemeurherve could someone from the Jenkins community review this PR?

[lemeurherve]

I'm not a plugin maintainer, not sure my review would add a lot.

@cyrille-leclerc WDYT of adding the adopt-this-plugin tag to this repository?
@tigerquoll WDTY of adopting this plugin? Ref: https://www.jenkins.io/doc/developer/plugin-governance/adopt-a-plugin/

[cyrille-leclerc]

@lemeurherve I would love to make this library open for adoption.