Unexpectedly bad performance

cstore_fdw 1.6.0 on postgresql-10

So, I've got a lot of time-series data in a table that looks like this:

```
spider=> create foreign table history_y2017 (
itemid bigint not null,
clock integer not null,
value numeric(20,0) not null,
ns integer default 0 not null,
constraint check_clock CHECK ( clock >= 1483228800 AND clock < 1514764800)
)
SERVER cstore_server OPTIONS(compression 'pglz');
insert into history_y2017 select * from history where clock >= 1483228800 AND clock < 1514764800 order by itemid,clock,ns;
analyze history_y2017;
```

```
spider=> explain analyze verbose select * from history_y2017 where itemid=1464 and clock 1497582092 and clock <= 1497668492;
QUERY PLAN
------------------------------------------------------------------------------------------------------------------------------------
Foreign Scan on public.history_y2017 (cost=0.00..12087188.56 rows=851 width=20) (actual time=995.497..1276.525 rows=1440 loops=1)
Output: itemid, clock, value, ns
Filter: ((history_y2017.clock > 1497582092) AND (history_y2017.clock <= 1497668492) AND (history_y2017.itemid = 1464))
Rows Removed by Filter: 28560
CStore File: /var/lib/pgsql/10/data/cstore_fdw/16385/17191
CStore File Size: 3549773603
Planning time: 0.408 ms
Execution time: 1278.834 ms

```

Benchmarking it on a select benchmark gives me ~3 tps...

Changing the block_row_count from default to '1000' only serves to make the matter four times worse.