[pvdata-core] base class for implementing services

[Marty K. <mrk...@co...>]

In May I visited BNL to discuss my priorities now the v3Channel which 
implements a version of Channel that provides pvAccess to V3 records.

My priority now is to help Goubao start implementing services that run 
in pvIOCCP.

Since it will take a lot of effort to implement support for V4 records 
that includes support and record processing it means that a service must 
do more work than services in javaIOC. The services in javaIOC are 
implemented via support attached to a field of a V4 record which has 
fields related to the service.

A base class for Channel now exists that implements some of the common 
things that must be done to implement Channel.

v3Channel was modified to use this base class. I have tested and 
committed what has been done so far.

There is still more work to do on this base class before it is ready for 
use.

BUT there are still problems and I need Matej to look and see if he can 
help.

I have created an exampleChannel that extends PVServiceBase.

When /home/mrk/hg/pvIOCCPP/iocBoot/testV3Channel is started the 
exampleChannel is stated that supports a single channel named 
exampleChannel.

To see what is wrong do the following:

start the IOC via

valgrind ../../bin/linux-x86/testV3Channel st.cmd

valgrind will show what is wrong

On the javaIOC swtshell ask for get

1) connect to exampleChannel.
    It does connect but then immediately disconnect
    valgrind will show what is wrong.

2) again start the IOC (not necessary to run under valgrid)
    connect get to exampleChannel
    click createGet

Note the message on the IOC console.
They are coming from pvIOCCPP/pvIocApp/exampleService/exampleChannelGet.cpp

It never returns from
     channelGetRequester->channelGetConnect(
        Status::OK,
        getPtrSelf(),
        pvTop,
        bitSet);

Any ideas?

Marty