Large refactoring and sdmx support for codelist (see details)

refactoring:

retracted RtmsCodelistFinder as it duplicates the role of
CsvCodelistImporter. moved functionality from former to latter.
retracted FigisConnectionManager and placed connection pool management
in RtmsConfiguration, where it naturally belongs.
recast CodelistService to CodelistImporter to match VR terminology
retracted CodelistAccessor and moved iteration functionality directly in
CodelistImporter to keep it 
inherited Concepts and Attributes from MetadataElements rather than
AbstractAssets. This eliminate the dangerous dependency on VR classes
that were not meant to be extended by plugins.
recast AbstractService as MetadataBrowser, to clarify  and better match
VR terminology.
focused MetadataBrowser on metadata retrieval and introduced a separate
RtmsMetadata to model metadata results 
MetadataBrowser is now handled internally by RtmsConfiguration, which
exposes the RtmsMetadata to the rest of the plugin.
retracted AbstractServiceFactory as no longer needed under the setup
above.
moved all constants in a dedicates Constants class, which is good
practice when constants are publicly shared.
overall, retracted dependencies on singleton pattern for more rational
object wiring and easier testing.
overall, removed used and propagation of of checked exception in favour
of unchecked exceptions.
swapped role of 'endpoint' and 'url' in configuration, as per VR
terminology
eliminated unused code paths and methods
concentrated codelist support in top-level package, metadata handling in
metadata package.
plenty of small changes throughout to improve flow and understandibility
introduced Utlls class with helpers for JDB resource handling
proper connection management and JDBC resources cleanup
timed logging on remote ops

behavioural changes:

RtmsConfiguration now allows to refresh metadata (creates and uses a new
MetadataBrowser), a property used internally to short-circuit this.
RtmsBrowser asks for a refresh() systematically.
added isCode() on RtmsAttributes, currently still on the basis of
name-based heuristics
in table transformation, columns that do no have values are omitted from
rows
in table transformation, concepts that have no code columns do not
generate rows

sdmx support:

Browser now supports discovery of SdmxCodelists assets;
added SdmxCodelistImporter and Table2RDF to allow codelist retrieval in
Sdmx
Merge branch 'master' of
https://github.com/virtual-repository/virtual-rtms.git