add nio ssl as explicit dep

By adding a handler that retains self on the channel's close future, the connection gets retained until the channel is closed. We break the retain cycle by using a weak reference to self. The completion handler in `send(:onResponse:)` is enough to ensure that the connection stays alive while a message is sent to the server.

* Add test for empty keyed encoding

* Replace _KeyedError with a flag that is set as soon as a keyed container is requested

The encoder will fallback to JSONB using JSONEncoder even if the encoded type does not encode any key.

* psql point endianness, fixes #125

* add test to allTests

* fix allTests

* added polygon data-type

* added polygon data-type

* added tests for polygon

* fixed polygon typos

It seems timestamp values are adjusted for the local time zone.
When converting to a Date, we need to subtract the local offset to get
the correct value.
When converting from a Date, it is better to use the timestamptz type,
which works with both kinds of fields and does not have this problem.

…117)" (#132)

This reverts commit bb4e603.

* configure performance testing

* add -enable-testing

* use swift 4.1 + add psql to perf test

* Significantly improve performance of row decoding.

Motivation:

1. `PostgreSQLRowDecoder`: `_KeyedDecodingContainer.decode` is called once for each key on the decoded object. With the current implementation, this performs a linear search on the `row` dictionary, which results in a total runtime (per object) of O(keys * columns), i.e. quadratic runtime ~O(keys^2). We replace the current linear search with one or two dictionary lookups if `tableOID != 0`, resulting in linear runtime (per object) in the number of keys (provided dictionary lookups can be assumed to take roughly constant time).
2. `PostgreSQLConnection.TableNameCache`: Most lookups are `tableName -> OID`. We accelerate that lookup by preparing a dictionary for that kind of lookup ahead of time, again replacing linear search.

Effect:

The time required for decoding ~5k objects with 9 fields each drops from ~0.4s on a Core i7-6700k (Release build) to ~0.2s, effectively doubling throughput. Optimization 1 contributes ~130 ms, Optimization 2 contributes ~70ms.

* Whitespace fixes.

* Comment fix.

* More whitespace, sorry.

* Implement `decodeIfPresent` to avoid two dictionary lookups per call.

* Minor code simplification.

* Add crashing test for decoding an array with NULL values

* Fix crash on NULL values inside arrays

ParameterStatus messages may be generated in response to a SET command or when the administrator sends the SIGHUP signals to the server. They will appear in the query response and should be ignored.