Restructure lazy query API

It doesn't make any sense to limit lazy queries to statments *prepared*
inside a transaction. We really only care that we're in a transaction
when the statement is executed. This does introduce a new runtime error
if a statement prepared on connection A is executed on a transaction
prepared on connection B, but I don't think anyone will *ever* run into
that.

Author: sfackler

README.md

@@ -17,7 +17,7 @@ extern crate time;
 
 use time::Timespec;
 
-use postgres::{PostgresConnection, PostgresStatement, NoSsl};
+use postgres::{PostgresConnection, NoSsl};
 use postgres::types::ToSql;
 
 struct Person {

src/error.rs

@@ -506,6 +506,8 @@ pub enum PostgresError {
     /// The communication channel with the Postgres server has desynchronized
     /// due to an earlier communications error.
     PgStreamDesynchronized,
+    /// A prepared statement was executed on a connection it does not belong to
+    PgWrongConnection,
 }
 
 impl PostgresError {
@@ -516,7 +518,11 @@ impl PostgresError {
             PgStreamError(ref err) => format!("{}", *err),
             PgStreamDesynchronized =>
                 ~"The communication stream with the Postgres server has \
-                  become desynchronized due to an earlier communications error"
+                  become desynchronized due to an earlier communications \
+                  error",
+            PgWrongConnection =>
+                ~"A statement was executed on a connection it was not \
+                  prepared on "
         }
     }
 }