MDEV-20611: MRR scan over partitioned InnoDB table produces "Out of memory" error

Fix partitioning and DS-MRR to work together

- In ha_partition::index_end(): take into account that ha_innobase (and
  other engines using DS-MRR) will have inited=RND when initialized for
  DS-MRR scan.
- In ha_partition::multi_range_read_next(): if the MRR scan is using
  HA_MRR_NO_ASSOCIATION mode, it is not guaranteed that the partition's
  handler will store anything into *range_info.
- In DsMrr_impl::choose_mrr_impl(): ha_partition will inquire partitions
  about how much memory their MRR implementation needs by passing
  *buffer_size=0. DS-MRR code didn't know about this (actually it used
  uint for buffer size calculation and would have an under-flow).
  Returning *buffer_size=0 made ha_partition assume that partitions do
  not need MRR memory and pass the same buffer to each of them.

  Now, this is fixed. If DS-MRR gets *buffer_size=0, it will return
  the amount of buffer space needed, but not more than about
  @@mrr_buffer_size.

* Fix ha_{innobase,maria,myisam}::clone. If ha_partition uses MRR on its
  partitions, and partition use DS-MRR, the code will call handler->clone
  with TABLE (*NOT partition*) name as an argument.
  DS-MRR has no way of knowing the partition name, so the solution was
  to have the ::clone() function for the affected storage engine to ignore
  the name argument and get it elsewhere.

mysql-test/include/partition_mrr.inc

@@ -0,0 +1,46 @@
+--source include/have_partition.inc
+
+--disable_warnings
+drop table if exists t1,t3;
+--enable_warnings
+
+--echo #
+--echo # MDEV-20611: MRR scan over partitioned InnoDB table produces "Out of memory" error
+--echo #
+create table t1(a int);
+insert into t1 values (0),(1),(2),(3),(4),(5),(6),(7),(8),(9);
+
+set @tmp=@@storage_engine;
+eval set storage_engine=$engine_type;
+
+create table t3 (
+  ID bigint(20) NOT NULL AUTO_INCREMENT,
+  part_id int,
+  key_col int,
+  col2 int,
+  key(key_col),
+  PRIMARY KEY (ID,part_id)
+) PARTITION BY RANGE (part_id)
+(PARTITION p1 VALUES LESS THAN (3),
+ PARTITION p2 VALUES LESS THAN (7),
+ PARTITION p3 VALUES LESS THAN (10)
+);
+
+show create table t3;
+set storage_engine= @tmp;
+
+insert into t3 select 
+  A.a+10*B.a,
+  A.a,
+  B.a,
+  123456
+from t1 A, t1 B;
+
+set optimizer_switch='mrr=on';
+--replace_column 9 #
+explain 
+select * from t3 force index (key_col) where key_col < 3;
+select * from t3 force index (key_col) where key_col < 3;
+
+drop table t1,t3;
+