Re-order Connection's RPC methods to match protobuf spec.
Rename 'Connection.rollback_transaction' -> 'rollback'.

Makes it consistent w/ 'commit', as well as matching the protobuf RPC.
Implement 'allocate_ids' RPC on Connection.

Fixes #272.
Note protobuf RPCs mapped by connection methods.

Remove section header comments, per @dhermes feedback.
Merge pull request #299 from tseaver/272-add-allocate_ids-rpc

Fix #272: add 'allocate_ids' RPC to connection