Improvements to binding

- We were allocating and managing bound parameter values and format codes lists
  inside NpgsqlBind. Refactored this into NpgsqlParameter - each parameter manages
  their own binding data, reduces allocations and simplifies code.
- Removed NpgsqlBind class (see previous commit)
- Relates to #308
Removed class for NpgsqlQuery

- Removed class instantiation for NpgsqlQuery message (see previous commits).
- New QueryManager handles the details of writing queries to the wire and to
  pre-build some commonly-used queries for efficiency.
- Relates to #308