Connection Pool and cleanup

Connection Pool:
- Connection pools (#499)
- Connection pools documentation and example (#533)

Smaller features:
- Add connection methods is_connected() and ping_server() (#528)
- Add sqlpp::compat::make_unique (#527)

Cleanup:
- Multiple code cleanups, in particular date/time code (e.g. #509, #511, #514, #517, #520)
- Replace include guards with "#pragma once" (#506)

Fix brace initialization, #498

Bugfixes and cleanups

Many fixes, including

Core & Connectors:
   - Added `OR REPLACE` clause.
   - Be stricter about not mixing aggregate and non-aggregate expressions
   - Parameters for `HAVING`
   - Multi-row `INSERT OR IGNORE`
   - Corrected parenthesis for several functions
   - Allow `DISTINCT` in `MIN` and `MAX`
   - Added `UPPER` and `LOWER`
   - #471 serializers: ensure float precision (#472)

ddl2ccp
  - Partial rewrite of ddl2cpp including some unit tests
  - Support for MariaDB GENERATED and PERIOD

Documentation
  - multiple updates

Massive refactor and cleanup release

The changes below require git/build rules to be changed.
I am expecting the result to be considerably simpler, though.
It will also be much simpler to make changes to the library or
individual connectors now (most of the misc changes below would
have been much harder to do with the former multiple repos).

Some of the misc changes below might break existing code in very
rare cases (I am aware of exactly one).
Happy to help fixing those :-)

Connector libraries
 - Moved connector libraries for mysql/postgresql/sqlite3 into the sqlpp11 repository
 - Made connector libraries header-only
 - Added `USE_SYSTEM_DATE` cmake option (#372)

Documentation:
 - Created docs directory (#364)
 - Removed wiki pages
 - Document multi_insert for time_point columns (#367)

Fixes:
 - Several DateTime fixes for mysql
 - Add order_by and limit for mysql remove and update
 - Added shift left and shift right operators.
 - Added blob support for postgresql
 - Support MySQL connect timeout option

Misc:
 - Replace serializer_t
 - Remove multi_column.
 - Remove tvin
 - Remove rhs_wrap
 - Remove _is_trivial from *_operand
 - Add is_equal_or_null(col, some_value_or_null)
 - Remove variations of serialization from interpretable
 - Remove null_is_trivial_value
 - Remove table::ref (see #345).
 - Remove same-name check for result columns

Release 0.60

* Add date as a dependency, make fetch friendly
* Update travis
* fixed #342: ambiguous comparison operators (c++20)
* Add support for parameterized_verbatim
* Add some MYSQL types to the ddl2cpp script

Release 0.59

Add -identity-naming argument from ddl2cpp to sqlite2cpp.py
Add wishlist.md
Add vcpkg installation instructions
Add minimal over() implementation for aggregate functions
Multiple bug fixes

Various enhacements

 - new data types for ddl2cpp
 - new option to generate one header file per table
 - support for string_view
 - support for INSERT INTO from SELECT
 - multiple examples

Maintenance release

Fix merge error in 0.55

Blob support, multiple new datatypes and small fixes

- ddl2cpp:
  - add multiple data types
  - option to not camel-case SQL names in C++
  - allow digits in names
- Allow tuples to be passed to insert_set
- Fix some static_assert messages