Batch large-table loads to bound memory and add per-table state to stats

tests/test_coerce.py

@@ -10,11 +10,19 @@ from sqlmem.cache import CacheManager
 
 class _FakeCursor:
     def __init__(self, rows):
-        self._rows = rows
+        self._rows = list(rows)
+        self._pos = 0
         self.description = None
 
     def fetchall(self):
-        return self._rows
+        out = self._rows[self._pos :]
+        self._pos = len(self._rows)
+        return out
+
+    def fetchmany(self, size):
+        out = self._rows[self._pos : self._pos + size]
+        self._pos += len(out)
+        return out
 
 
 class FakeSource: