Allow cypher to read data and modify schema in the same transaction.

This is achieved by delaying the choice of statement type (data write vs schema write)
until specific operations are required.
Remove concept of QueryType from Cypher.
All kernel operations are accessed through a single Statement API which in turn
exposes narrower interface for read, data write or schema write operations.
Statement objects need to be closed, but indivdual operations objects don't have any
additonal state and so don't need to be closed.
Extract narrower interface for Statement.

Hides internal state from clients of the kernel.
Return Statement instead of its implementation.

For better encapsulation.