Avoid capturing Request in LogRecord parameters

[jglick]

Noticed in a heap dump that there were several Channels sticking around after the associated builds had completed, and found that they were referenced in a log ring buffer

this     - value: hudson.remoting.Channel #1
 <- channel     - class: hudson.remoting.RemoteClassLoader$ClassLoaderProxy, value: hudson.remoting.Channel #1
  <- classLoaderProxy     - class: hudson.remoting.UserRequest, value: hudson.remoting.RemoteClassLoader$ClassLoaderProxy #1
   <- [0]     - class: java.lang.Object[], value: hudson.remoting.UserRequest #1
    <- parameters     - class: java.util.logging.LogRecord, value: java.lang.Object[] #97789
     <- [177]     - class: java.util.logging.LogRecord[], value: java.util.logging.LogRecord #32
      <- records     - class: hudson.WebAppMain$1, value: java.util.logging.LogRecord[] #1
       <- handler     - class: hudson.WebAppMain, value: hudson.WebAppMain$1 #1
        <- [4]     - class: java.lang.Object[], value: hudson.WebAppMain #1
         <- elementData     - class: java.util.ArrayList, value: java.lang.Object[] #1868
          <- _destroyServletContextListeners     - class: winstone.HostConfiguration$1, value: java.util.ArrayList #91
           <- _context (Java frame)     - class: org.eclipse.jetty.webapp.WebAppClassLoader, value: winstone.HostConfiguration$1 #1

whose message I traced to this line.

See jenkinsci/jenkins#6643 for background.