REF: concat on bm_axis==0
Makes it so we no longer need `indexers` in `JoinUnit` (and in fact, so that JoinUnit is only used for axis==1)

```
asv continuous -E virtualenv -f 1.01 master HEAD --append-samples --record-samples -b join_merge
[...]
       before           after         ratio
     [26064f0f]       [5dedf170]
     <master>         <ref-concat-axis0>
-      15.8±0.9ms       15.0±0.4ms     0.95  join_merge.MergeAsof.time_on_int('backward', None)
-      16.8±0.4ms       15.9±0.3ms     0.94  join_merge.MergeAsof.time_on_int('forward', None)
-      19.6±0.9ms       18.5±0.2ms     0.94  join_merge.MergeAsof.time_on_int('nearest', None)
-      16.1±0.4ms       15.1±0.3ms     0.94  join_merge.MergeAsof.time_on_int('backward', 5)
-      17.8±0.9ms       16.7±0.4ms     0.94  join_merge.MergeAsof.time_on_int32('backward', None)
-        84.4±6ms         78.6±2ms     0.93  join_merge.MergeAsof.time_by_object('backward', 5)
-        83.1±3ms         77.3±2ms     0.93  join_merge.MergeAsof.time_by_object('backward', None)
-        20.1±1ms       18.6±0.5ms     0.93  join_merge.MergeAsof.time_on_uint64('nearest', 5)
-      20.2±0.8ms       18.7±0.3ms     0.93  join_merge.MergeAsof.time_on_int('nearest', 5)
-        17.5±1ms       16.2±0.3ms     0.92  join_merge.MergeAsof.time_on_int('forward', 5)
-        22.3±2ms       20.5±0.4ms     0.92  join_merge.MergeAsof.time_on_int32('nearest', 5)
-      18.2±0.4ms       16.8±0.4ms     0.92  join_merge.MergeAsof.time_on_int32('backward', 5)
-        21.8±1ms       19.9±0.4ms     0.91  join_merge.MergeAsof.time_on_int32('nearest', None)
-      19.0±0.9ms       17.2±0.3ms     0.91  join_merge.MergeAsof.time_on_int32('forward', None)
-        17.6±1ms       15.9±0.6ms     0.90  join_merge.MergeAsof.time_on_uint64('forward', 5)
-        335±10ms          300±9ms     0.89  join_merge.Merge.time_merge_dataframes_cross(False)
-         334±7ms         299±10ms     0.89  join_merge.Merge.time_merge_dataframes_cross(True)
-        19.4±2ms         17.2±1ms     0.89  join_merge.Join.time_join_dataframe_index_multi(True)
-        17.1±1ms       15.1±0.2ms     0.88  join_merge.MergeAsof.time_on_uint64('backward', None)
-        10.3±1ms       8.97±0.4ms     0.87  join_merge.Join.time_join_dataframe_index_shuffle_key_bigger_sort(True)
-      19.9±0.7ms       17.4±0.4ms     0.87  join_merge.MergeAsof.time_on_int32('forward', 5)
-        18.6±1ms       15.9±0.2ms     0.86  join_merge.MergeAsof.time_on_uint64('forward', None)
-        8.44±1ms       7.20±0.7ms     0.85  join_merge.Join.time_join_dataframe_index_single_key_small(False)
-      1.98±0.09s       1.58±0.04s     0.80  join_merge.JoinIndex.time_left_outer_join_index
-      9.80±0.8ms       7.61±0.8ms     0.78  join_merge.Join.time_join_dataframe_index_single_key_bigger(False)
-      9.39±0.5ms       7.25±0.4ms     0.77  join_merge.Concat.time_concat_small_frames(1)
-        221±20ms          170±4ms     0.77  join_merge.MergeCategoricals.time_merge_cat
-        439±10ms          300±9ms     0.68  join_merge.MergeCategoricals.time_merge_object
```
nice!