Work on groups

Some dev/testing API calls are now turned off by default in production mode,
 see "restrictApi" settings entry, returning NON_PRODUCTION API error.

Corrections to how account's defaultGroupId works, removing "effective groupID"
 which overly complicated matters.
In relation to above, DEFAULT_GROUP (0) no longer exists and NO_GROUP(-1) now has
 the value 0 instead.
So transactions can no longer have txGroupId of DEFAULT_GROUP, which in turn
 required all the erroneous "effective groupID" code.

API call /addresses/{address} now supplies blockchain-wide defaultGroupId if
 account doesn't exist or if account's default not set and NO-GROUP not allowed.

API /transactions/pending now offloaded to repository instead of Java-based
 processing and filtering.

Transaction approval checks added to Block.isValid

Groups now have min/max approval block delays.
 Checks added to incoming unconfirmed, block generator, block.isValid, etc.

'needing approval' and 'meets approval threshold' now split into separate calls.

NB: settings.json no longer part of git repo

src/main/resources/i18n/ApiError_en.properties

@@ -7,6 +7,7 @@ NO_BALANCE=not enough balance
 NOT_YET_RELEASED=that feature is not yet released
 UNAUTHORIZED=api call unauthorized
 REPOSITORY_ISSUE=repository error
+NON_PRODUCTION=This API call is not permitted for production systems
 
 # Validation
 INVALID_SIGNATURE=invalid signature