These were previously in the appenders chapter, now moved to the
receivers chapter, and refined a bit to make them a bit easier to
follow.

Probably want to consider inserting the chapter on receivers
somewhere near the middle...

The Appenders chapter had become quite long due to the inclusion
of the documentation of Receiver components.

This branch refactors the previously contributed "server" and
"remote" components such that they now extend a common "receiver"
component.  A receiver is a component that receives logging
events from a remote appender.  A connection between and an
appender and a receiver may be initiated in either direction;
i.e. a receiver may initiate the connection to an appender or
vice-versa.  The receiver component is configured in Joran using
a <receiver> element.  The class specified in this element
determines whether the receiver will passively await connections
from remote appenders (i.e. as in the legacy SimpleSocketServer)
or whether it will initiate a connection to a remote appender
(a ServerSocketAppender).

Still needs lots of proofreading...

The subclass may need a reference to executor to pass to objects that
will need it later when the receiver is running.

This will make it easier to refactor SocketServer so that it extends
ReceiverBase.

The object that is composed with a ServerRunner already implements
LifeCycle.  Having a start method complicates the use of the runner
without adding any real value.

Also fixed up the taxonomy -- what was a SocketRemote is now a receiver
component that connects to a remote appender peer.

This will eliminate the need to add a nested component rule for each new
SSL component.

ReceiverAction can instantiate a component that is a subtype of
ReceiverBase.  SocketRemote and SocketServer will be refactored to
extend the ReceiverBase type.

The existing SocketRemote and SocketServer classes will be refactored to
descend from this base class.

In preparation for introducing the concept of a "receiver" component
(which can be either a component that listens passively for incoming
connections from remote peers or one that actively connects to a remote
peer), this commit fixes up the taxonomy in the ServerSocketAppenderBase
implementation.  Now a ServerSocketAppender will have zero or more
connected remote receiver clients.

The MockAppender now uses a BlockingQueue to hold appended events
until the test subject can pick them up using either awaitAppend or
getLastEvent.

Just a first draft.  Please edit and/or pare down as appropriate.

components

This ensures that no queuing occurs when the number of threads is less
than or equal to the maximumPoolSize.  Also added constants for all of
the default values, and changed the defaults for maximumPoolSize and
keepAliveTime.