removed unused SocketReceiver.createExecutorService
use "receiver" instead of "remote" in receiver diagnostic messages
marked SocketAppender convenience constructors in access as deprecated

For consistency with logback-core.
moved MockContext and MockExecutorService to ..core.net.mock
added missing doc comments to MockExecutorService
fixed references to MockContext in test classes
removed old SocketAppenderTest from classic and access

These test classes were really testing SocketAppenderBase not
SocketAppender, per se.  The new implementation of SocketAppenderBase is
significantly different and will require a new unit test suite.
improved SocketAppenderBase

SocketAppenderBase now uses SocketConnector for its (re-)connection
logic, and uses an asynchronous task to dispatch events to the remote
receiver.  A configurable queue is utilized to relay logging events from
the append method to the dispatch task.  When the queue length is zero
(the default), a SynchronousQueue is utilized, preserving the previous
appender behavior.  When the queue length is greater than zero, a
bounded queue is utilized, allowing the appender to efficiently drop
logging events when remote receiver (or network) cannot keep up with the
rate of logging events delivered to the appender.
modified concrete appenders to use appropriate constructors

Mutable fields in SocketAppenderBase were previously exposed as
protected fields and modified by constructors in concrete subclasses.
This commit improves encapsulation of the base class by exposing
protected constructors for subclasses to invoke rather than directly
exposing the fields.  These new constructors are marked as deprecated,
as the convenience constructors on the base classes (which are also
deprecated) are the sole reason for their existence.  A future release
should remove all but the no-arg constructor from the base class and
subclasses.