Make read and write messages contiguous (#1387)

The `BlockIO` trait only allows us to access contiguous sets of blocks
for reading and writing.

However, our internal `Message` type converts these contiguous spans
into random-access block lists. This adds complexity: we have to support
the possibility for disjoint operations everywhere, even though they
should never actually happen.

This PR changes the `Message` types to accept a starting index + block
count (for reads), or a starting index + block and context data (for
writes). In addition, the `ReadResponse` is simplified to not include
per-block indexes, because they're implied by the start index + position
in the list.

Various tests for disjoint reads and writes are removed.

common/src/impacted_blocks.rs

@@ -129,6 +129,14 @@ impl ImpactedBlocks {
         ImpactedBlocks::Empty
     }
 
+    /// Returns the first impacted address
+    pub fn start(&self) -> Option<ImpactedAddr> {
+        match self {
+            ImpactedBlocks::InclusiveRange(a, _) => Some(*a),
+            ImpactedBlocks::Empty => None,
+        }
+    }
+
     /// Create a new ImpactedBlocks range starting at a given offset, and
     /// stretching n_blocks further into the extent. Panics an error if
     /// `extent_size` is 0. Returns ImpactedBlocks::Empty if n_blocks is 0.