This change set adds the following supporting structs for adding
support for verifying blocks pulled by onboarding in future CRs:

- Ledger interceptor: intercepts a commit of a block, and invokes
  a callback.

- VerificationRegistry: tracks commit of config blocks, and builds
  channelconfig bundles from them, in order to support verification
  of blocks pulled.

- BlockVerifierAssembler and BlockValidationPolicyVerifier: together
  they build block verifiers out of config blocks.

- verifierLoader: Loads a mapping of chainID->cluster.BlockVerifier,
  which is to be used at OSN startup to preload the existing verifiers.
  It is needed in cases we recover from a crash, or if we do
  dynamic onboarding and the previous config blocks have been committed
  to the ledger before the OSN was started.

In the next CR, I will wire all these into the onboarding infrastructure
itself, and they will be used to hold the latest bundle per channel
in order to verify block signatures.

Change-Id: Ic9fc99243baa5c2cef97103d001180207414d98a
Signed-off-by: yacovm <yacovm@il.ibm.com>