[Tile-serving] [openstreetmap/osm2pgsql] WIP: Experimental middle implementations (#1384)

[Jochen Topf]

This PR adds two new middle implementations:
- `db` is similar to `pgsql`. All data is stored in the database. A new table structure with JSONB for tags and relation members
  is used.
- `mem` is similar to `ram`. It only stores node locations and way node lists in RAM. Used for import-only workflows that do
  not need the flex-output two-stage processing.

These are **EXPERIMENTAL**! They are not well tested, not complete and performance might be horrible. They are intended to start a discussion about what we need and how to best implement it. This PR will evolve over time.

To enable these, set the environment variable `OSM2PGSQL_MIDDLE` to either `db` or `mem`. This setting is independent of the `--slim` setting! This way of setting the middle is temporary and gives us flexibility while experimenting, it will not be available in this form in the final implementation.
You can view, comment on, or merge this pull request online at:

  https://github.com/openstreetmap/osm2pgsql/pull/1384

-- Commit Summary --

  * Add rapidjson library
  * Add two new experimental middle implementations: db and mem

-- File Changes --

    M CMakeLists.txt (4)
    A contrib/rapidjson/README.contrib (2)
    A contrib/rapidjson/include/rapidjson/allocators.h (271)
    A contrib/rapidjson/include/rapidjson/document.h (2575)
    A contrib/rapidjson/include/rapidjson/encodedstream.h (299)
    A contrib/rapidjson/include/rapidjson/encodings.h (716)
    A contrib/rapidjson/include/rapidjson/error/en.h (74)
    A contrib/rapidjson/include/rapidjson/error/error.h (155)
    A contrib/rapidjson/include/rapidjson/filereadstream.h (99)
    A contrib/rapidjson/include/rapidjson/filewritestream.h (104)
    A contrib/rapidjson/include/rapidjson/fwd.h (151)
    A contrib/rapidjson/include/rapidjson/internal/biginteger.h (290)
    A contrib/rapidjson/include/rapidjson/internal/diyfp.h (258)
    A contrib/rapidjson/include/rapidjson/internal/dtoa.h (245)
    A contrib/rapidjson/include/rapidjson/internal/ieee754.h (78)
    A contrib/rapidjson/include/rapidjson/internal/itoa.h (304)
    A contrib/rapidjson/include/rapidjson/internal/meta.h (181)
    A contrib/rapidjson/include/rapidjson/internal/pow10.h (55)
    A contrib/rapidjson/include/rapidjson/internal/regex.h (701)
    A contrib/rapidjson/include/rapidjson/internal/stack.h (230)
    A contrib/rapidjson/include/rapidjson/internal/strfunc.h (55)
    A contrib/rapidjson/include/rapidjson/internal/strtod.h (269)
    A contrib/rapidjson/include/rapidjson/internal/swap.h (46)
    A contrib/rapidjson/include/rapidjson/istreamwrapper.h (115)
    A contrib/rapidjson/include/rapidjson/memorybuffer.h (70)
    A contrib/rapidjson/include/rapidjson/memorystream.h (71)
    A contrib/rapidjson/include/rapidjson/msinttypes/inttypes.h (316)
    A contrib/rapidjson/include/rapidjson/msinttypes/stdint.h (300)
    A contrib/rapidjson/include/rapidjson/ostreamwrapper.h (81)
    A contrib/rapidjson/include/rapidjson/pointer.h (1358)
    A contrib/rapidjson/include/rapidjson/prettywriter.h (255)
    A contrib/rapidjson/include/rapidjson/rapidjson.h (615)
    A contrib/rapidjson/include/rapidjson/reader.h (1879)
    A contrib/rapidjson/include/rapidjson/schema.h (2006)
    A contrib/rapidjson/include/rapidjson/stream.h (179)
    A contrib/rapidjson/include/rapidjson/stringbuffer.h (117)
    A contrib/rapidjson/include/rapidjson/writer.h (610)
    A contrib/rapidjson/license.txt (57)
    A contrib/rapidjson/readme.md (210)
    M src/CMakeLists.txt (2)
    A src/middle-db.cpp (856)
    A src/middle-db.hpp (137)
    A src/middle-mem.cpp (123)
    A src/middle-mem.hpp (91)
    M src/middle.cpp (26)

-- Patch Links --

https://github.com/openstreetmap/osm2pgsql/pull/1384.patch
https://github.com/openstreetmap/osm2pgsql/pull/1384.diff

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