dqlite.h: Support switching off the auto-bootstrap configuration

[freeekanayaka]

Add the ability to also turn off the automatic bootstrap behavior.

This makes dqlite_server_set_auto_bootstrap a bit more complete and consistent with the other dqlite_server_set_xxx APIs which allow to override/reset previous calls.

Since this code hasn't been part of a release yet and hasn't any consumer that we know of, I think it's safe to merge the change if desired.

[codecov[bot]]

Codecov Report

Merging #520 (33075f2) into master (bf029c8) will decrease coverage by 0.06%. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #520      +/-   ##
==========================================
- Coverage   61.44%   61.38%   -0.06%     
==========================================
  Files          34       34              
  Lines        6813     6813              
  Branches     2024     2024              
==========================================
- Hits         4186     4182       -4     
- Misses       1342     1344       +2     
- Partials     1285     1287       +2     

Impacted Files	Coverage Δ
src/server.c	59.19% <100.00%> (ø)

... and 1 file with indirect coverage changes