Implement count command

[maheshrajamani]

Implement count command to return number of documents in a collection based on the filter condition.
Filter clause is extension on what is used for other commands like find, delete, update, etc.
The count command will use CQL statement as
select count(key) from collection where (based on filter).

Response of the count to be returned as
{
"status": {
"counted_documents": 2
}
}