Implement live read replication

This commit adds an http server and client for streaming snapshots
and WAL pages from an upstream Litestream primary to a read-only
replica.

testdata/wal-writer/live/README.md

@@ -0,0 +1,19 @@
+WAL Writer Live
+=================
+
+This test is to ensure we can copy a WAL file into place with a live DB and
+trigger a checkpoint into the main DB file.
+
+To reproduce the data files:
+
+```sh
+$ sqlite3 db
+
+sqlite> PRAGMA journal_mode = 'wal';
+sqlite> CREATE TABLE t (x);
+sqlite> PRAGMA wal_checkpoint(TRUNCATE);
+sqlite> INSERT INTO t (x) VALUES (1);
+
+sqlite> CTRL-\
+```
+