Made methods in IDManager final for better inlining
Fixes #260
Added TransactionBuilder interface which gives the user the ability to configure transactions prior to opening them and thereby controlling performance and consistency checking aspects of the transaction on a per transaction basis. Use TitanGraph.buildTransaction() for that.
One aspect TransactionBuilder lets you control is whether a TitanTransaction should verify the existence of all vertices internally. Since this is expensive, its disabled by default but can selectively be enabled via TransactionBuilder in the face of data degradation  on eventually consistent storage backends.
Fixes #363. Fixes #214.