Organization, cleanup, and optimization

The Flush command is never used anymore, and it is easily confused with Stream.Flush(); remove support for it.
Simplify and mske more consistent some of the query pathways.
Avoid calling Stream.Flush() in multiple locations; call it in ProcessBackendResponses() instead.
Remove a bunch of redundant/unused code.
The two Process*BackendResponse*() functions are now the only entry points into that functionality.
NopgsqlQuery now constructs with a connector and query text (byte[] or string).
The "unknown" type supports binary decoding now.
cancelRequestCalled parameter is never set true; remove it.
Optimize ExecuteBlind() a little more.
Make sure to block the notification thread before writing query messages to the wire.
Revert addition of redundant using (m_Connector.BlockNotificationThread()) from last commit.
Add support for "Always Prepare"

Add connection string value "AlwaysPrepare".
Implement "AlwaysPrepare" support on NpgsqlCommand.
Refactor Prepare code so that calling Prepare() from within GetReader() avoids redundant sanity checks.
When a previously prepared statement command text is changed, automatically re-Prepare (if needed) before the next Execute.
Complete implementation of "ApplicationName" connection string value (I hope).
Hide enum DescribeTypeCode properly.
Optimization

Don't save lastRowDescription in the backend response handlers; save it in DataReader instead.
Send Portal Describe only once; save the row description for subsequent Executes.
Use a more efficient path to DataRow disposal on protocol version V3.
Shorten plan and portal names for a small network bandwidth savings.
Interlocked.Increment not needed for plan and portal number increments.
Bump the buffer size on BufferedStream to 8192.
Error handling

Remove incorrect usage of the Sync message - its purpose is for directing PG in case there's an error, not recovering from an error after it happens.
Remove useless try/catch constructs.
PG doesn't send a row description in response to Portal Describe if there's nothing to describe; handle this scenario properly.
Optimize NpgsqlExecute and NpgsqlSync just a bit more.
Refactor NpgsqlFlush and NpgsqlSync, minor clean up.