reftable/block: rename block_reader_start()

The function `block_reader_start()` does not really apply to the block
reader, but to the block iterator. It's name is thus somewhat confusing.
Rename it to `block_iter_seek_start()` to clarify.

We will rename `block_reader_seek()` in similar spirit in the next
commit.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

reftable/block_test.c

@@ -69,7 +69,7 @@ static void test_block_read_write(void)
 
 	block_reader_init(&br, &block, header_off, block_size, GIT_SHA1_RAWSZ);
 
-	block_reader_start(&br, &it);
+	block_iter_seek_start(&it, &br);
 
 	while (1) {
 		int r = block_iter_next(&it, &rec);