Merge branch 'ps/reftable-reusable-iterator'

Code clean-up to make the reftable iterator closer to be reusable.

* ps/reftable-reusable-iterator:
  reftable/merged: adapt interface to allow reuse of iterators
  reftable/stack: provide convenience functions to create iterators
  reftable/reader: adapt interface to allow reuse of iterators
  reftable/generic: adapt interface to allow reuse of iterators
  reftable/generic: move seeking of records into the iterator
  reftable/merged: simplify indices for subiterators
  reftable/merged: split up initialization and seeking of records
  reftable/reader: set up the reader when initializing table iterator
  reftable/reader: inline `reader_seek_internal()`
  reftable/reader: separate concerns of table iter and reftable reader
  reftable/reader: unify indexed and linear seeking
  reftable/reader: avoid copying index iterator
  reftable/block: use `size_t` to track restart point index

reftable/reftable-stack.h

@@ -66,6 +66,24 @@ int reftable_stack_add(struct reftable_stack *st,
 					  void *write_arg),
 		       void *write_arg);
 
+struct reftable_iterator;
+
+/*
+ * Initialize an iterator for the merged tables contained in the stack that can
+ * be used to iterate through refs. The iterator is valid until the next reload
+ * or write.
+ */
+void reftable_stack_init_ref_iterator(struct reftable_stack *st,
+				      struct reftable_iterator *it);
+
+/*
+ * Initialize an iterator for the merged tables contained in the stack that can
+ * be used to iterate through logs. The iterator is valid until the next reload
+ * or write.
+ */
+void reftable_stack_init_log_iterator(struct reftable_stack *st,
+				      struct reftable_iterator *it);
+
 /* returns the merged_table for seeking. This table is valid until the
  * next write or reload, and should not be closed or deleted.
  */