Timescaledb: 0.10.0 Release

High-level changes * Planning time improvement (up to 15x) when a hypertable has many chunks by only expanding (and taking locks on) chunks that will actually be used in a query, rather than on all chunks (as was the default PostgreSQL behavior). * Smarter use of HashAggregate by teaching the planner to better estimate the number of output rows when using time-based grouping. * New convenience function for getting the approximate number of rows in a hypertable (hypertable_approximate_row_count). * Fixed support for installing extension into non-public schemas * Other bug fixes and refactorings.

Notable commits * [12bc117] Fix static analyzer warning when checking for index attributes * [7d9f49b] Fix missing NULL check when creating default indexes * [2e1f3b9] Improve memory allocation during cache lookups * [ca6e5ef] Fix upserts on altered tables. * [2de6b02] Add optimization to use HashAggregate more often * [4b4211f] Fix some external functions when setting a custom schema * [b7257fc] Optimize planning times when hypertables have many chunks * [c660fcd] Add hypertable_approximate_row_count convenience function * [9ce1576] Fix a compilation issue on pre 9.6.3 versions

Thanks * @viragkothari for suggesting the addition of hypertable_approximate_row_count and @fvannee for providing the initial SQL used to build that function * 'mintekhab' from Slack for reporting a segfault when using upserts on an altered table * @mmouterde for reporting an issue where the extension implicitly expected to be installed in the public schema * @mfuterko for bringing some potential bugs to our attention via static analysis