This commit moves hyperloglog aggregates and functions to catalog as
well as migrates any hyperloglog related functions into
`src/backend/utils`.

We had significant deduplication in hyperloglog extension and utility
library that we use in the analyze related code. This commit removes the
deduplication as well as significant amount dead code. It also fixes
some compiler warnings and some coverity issues.

This commit also puts the hyperloglog functions in a separate schema
which is non-modifiable by non superusers.
Signed-off-by: NBhuvnesh Chaudhary <bchaudhary@pivotal.io>

- Port the hyperloglog extension into the contrib directory and make
corresponding makefile changes to get it to compile.
- Also modify initdb to install the HLL extension as part of gpinitsystem.
Signed-off-by: NOmer Arap <oarap@pivotal.io>
Signed-off-by: NEkta Khanna <ekhanna@pivotal.io>