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

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+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-\
+```
+

testdata/wal-writer/static/README.md

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+WAL Writer Static
+=================
+
+This test is to ensure that WALWriter will generate the same WAL file as
+the `sqlite3` command line.
+
+To reproduce the data file:
+
+```sh
+$ sqlite3 db
+
+sqlite> PRAGMA journal_mode = 'wal';
+
+sqlite> CREATE TABLE t (x);
+
+sqlite> INSERT INTO t (x) VALUES (1);
+
+sqlite> CTRL-\
+```
+
+then remove the db & shm files:
+
+```sh
+$ rm db db-shm
+```
+