feat: Add a way to follow a SQL db instead of indexing

[Yawning]

This should allow multiple non-archival instances to share the same
database, with one and only one instance writing to the db.

Fixes: #84

[codecov]

Codecov Report

Merging #284 (551d26e) into main (6f19a05) will decrease coverage by 1.83%.
The diff coverage is 2.98%.

@@            Coverage Diff             @@
##             main     #284      +/-   ##
==========================================
- Coverage   59.84%   58.01%   -1.84%     
==========================================
  Files          35       35              
  Lines        3990     4118     +128     
==========================================
+ Hits         2388     2389       +1     
- Misses       1397     1525     +128     
+ Partials      205      204       -1     

Impacted Files	Coverage Δ
conf/config.go	43.47% <ø> (ø)
main.go	8.42% <0.00%> (ø)
indexer/indexer.go	30.25% <3.00%> (-16.48%)	⬇️

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