Removed following properties of ColumnFamilyOptions, which have been deprecated for some time. (They cause issues with Cython right now, and will be obsolete in v7 anyway.)

* `soft_rate_limit`
* `hard_rate_limit`
* `rate_limit_delay_max_milliseconds`
* `purge_redundant_kvs_while_flush`
* `skip_log_error_on_recovery`

Remove unnecessary delete

Revert deleting Close method call

Add dependency to Cython in pyproject.toml file

Update tox configuration for unit test

Remove deprecated setup_requires

While `delete rocks_ptr` is a deterministic operation in C++, the Python analogue `del rocks_handle` is not – disposal and finalization of the Rocks database are entirely dependent on the Python garbage collector (q.v. the `python-rocksdb` tests themselves, which call `gc.collect()` after `del rocks_handle` to attempt to force the destructor to run). This change exposes a method to trigger the destruction of the underlying Rocks database pointer (deterministic!) through the Python Rocks handle; existing code will not need to be changed, as the Python object destructor (non-deterministic!) will now call this method.

This is the second revision of this PR – it resolves the first revision, #39.

Add support for Column Families in a runtime safe way.
Add unittests to test functionality
Insure all unittests are passing.
Cleaned up unittests to not use a fixed directory in tmp, but use tempfile

Merge options source_compaction_factor, max_grandparent_overlap_bytes and expanded_compaction_factor into max_compaction_bytes