redo security (aka. access control)

Break up monolithic SecurityPlugin API into three parts.

1. Authentication.
2. Authorization.
3. Access Control decision.  Left to ChannelProviders

src/client/pvAccess.cpp

@@ -8,9 +8,11 @@
 #include <epicsGuard.h>
 #include <pv/reftrack.h>
 #include <pv/valueBuilder.h>
+#include <pv/epicsException.h>
 
 #define epicsExportSharedSymbols
 #include <pv/pvAccess.h>
+#include <pv/security.h>
 
 namespace pvd = epics::pvData;
 
@@ -418,6 +420,11 @@ ChannelRequester::~ChannelRequester()
     REFTRACE_DECREMENT(num_instances);
 }
 
+PeerInfo::const_shared_pointer ChannelRequester::getPeerInfo()
+{
+    return PeerInfo::const_shared_pointer();
+}
+
 std::string DefaultChannelRequester::getRequesterName() { return "DefaultChannelRequester"; }
 
 void DefaultChannelRequester::channelCreated(const epics::pvData::Status& status, Channel::shared_pointer const & channel)