In order to support multiple copies of metadata objects in flight in AthenaMT
(required for running over more than one input file), such metadata objects
need to be stored into MetaCont - a container for objects of the same time
indexed by SID (file name).

This commit introduces a mechanism inside IOVDbMetaDataTool, which gets triggered
at BeginInputFile incident, and which stores IOVMetaDataContainer objects read
from each input file into MetaCont<IOVMetaDataContainer>, which is accessible
to clients in the MetaDataStore via ReadMetaHandle-s.

The changes affect several files:

* CondAttrListCollection.h

Introduced new overloaded version of the dump() which allows printing the contents
of CondAttrListCollection to MsgStream. This is mostly for debugging purpose.

* MetaCont.h

The interface was simplified by dropping several methods, that have no clients.
These mathods can be reintroduced later, if necessary.

* MetaHandleKey(.h,.icc)

The code was updated by analogy with the current implementation of CondHandleKey

* ReadMetaHandle.h

Dropped the inheritance from VarHandleBase, and fixed the implementation of the constructor.

* ReadMetaHandleKey.h

Introduced new constructor.

* IOVMetaDataContainer(.h,.cxx)

Simplified the merge() method and introduced new dump() method

* IOVPayloadContainer(.h,.cxx)

Introduced copy constructor and assignment operator

* IOVDbMetaDataTool(.h,.cxx)

Introduced two new methods fillMetaCont(), which adds IOVMetaDataContainer to MetaCont-s,
and dumpMetaConts()