Remove DQLITE_NEXT guard and unify with disk mode

[cole-miller]

This PR removes the DQLITE_NEXT build configuration. Code for the new disk mode that was previously guarded by #ifdef DQLITE_NEXT can instead be selected at run time by calling the new dqlite_node_create_v2 function. At the raft level, this is supported by making the "format version" number a dynamic value instead of a compile-time constant and plumbing it around.

Standalone version of the first part of #661, with the relevant review comments addressed (thanks @letFunny).

Signed-off-by: Cole Miller cole.miller@canonical.com

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 85.88235% with 24 lines in your changes missing coverage. Please review.

Project coverage is 77.87%. Comparing base (ee07596) to head (3e7e544). Report is 100 commits behind head on master.

Files with missing lines	Patch %	Lines
src/server.c	68.96%	6 Missing and 3 partials :warning:
test/raft/integration/test_uv_load.c	83.33%	6 Missing :warning:
src/raft/uv.c	72.72%	2 Missing and 1 partial :warning:
src/raft/uv_metadata.c	84.61%	0 Missing and 2 partials :warning:
src/raft/uv_segment.c	92.30%	0 Missing and 2 partials :warning:
src/raft/uv_encoding.c	90.00%	0 Missing and 1 partial :warning:
src/raft/uv_snapshot.c	50.00%	0 Missing and 1 partial :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #671 +/- ## ========================================== + Coverage 77.56% 77.87% +0.30% ========================================== Files 197 197 Lines 27638 27936 +298 Branches 5486 5520 +34 ========================================== + Hits 21438 21754 +316 - Misses 4326 4356 +30 + Partials 1874 1826 -48 ```

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[cole-miller]

As discussed on MM I've pushed commits to remove the flags argument to dqlite_node_create_v2, which now uses disk mode exclusively. dqlite_node_enable_disk_mode remains a no-op. This design removes ambiguity about when to migrate from one interface to another, makes the experimental nature of dqlite_node_create_v2 clearer, and leads to a better story for migrating to dqlite_node_create_v2 and eventually removing support fo removing the in-memory mode.