Enable additional MALLOC_CONF options on CI

On top of the production configuration, this commit adds two options:

* `abort:true` - most jemalloc warnings become fatal ([opt.abort])
* `junk:true` - both allocated and deallocated memory will be
  initialized to 0xa5. ([opt.junk])

Combined with the existing `abort_conf:true`, this should be equivalent
to enabling the `debug` feature of `jemallocator`.  We cannot enable
this feature in `Cargo.toml` because that would affect performance in
production.

[opt.abort]: http://jemalloc.net/jemalloc.3.html#opt.abort
[opt.junk]: http://jemalloc.net/jemalloc.3.html#opt.junk

Author: jtgeibel

.travis.yml

@@ -28,7 +28,7 @@ env:
     - PGPORT=5433
     - PATH=$HOME/.cargo/bin:$PATH
     - RUSTFLAGS="-C debuginfo=0 -D warnings"
-    - MALLOC_CONF=background_thread:true,abort_conf:true
+    - MALLOC_CONF=background_thread:true,abort_conf:true,abort:true,junk:true
 
 install:
   - sudo cp /etc/postgresql/10/main/pg_hba.conf /etc/postgresql/11/main/pg_hba.conf