Testability enhancement:  interfaces extracted from AerospikeClient and AsyncClient 

This change is motivated by the fact that it is hard to write unit tests  for functionality that uses client classes ie. DAO. Because of the fact that most API methods are marked as final it is not possible to 'mock' them for unit testing purposes. Introducing interfaces solves that issue and does not force developer to write wrapper/proxies

The change includes the following refactorings:
1) AerospikeClient renamed to DefaultAerospikeClient
2) AsyncClient renamed to DefaultAsyncClient
3) interface AerospikeClient extracted (with java docs pulled up)
4) interface AsyncClient extracted (with java docs pulled up)