Add DB tuning to keep conns option, and correct cache bypass on group lookup

[peterbroadhurst]

In PR chain with #593

From observations via pprof:

Excessive SQL connection overhead:

Cache bypass:

profile002.pdf

[codecov-commenter]

Codecov Report

Merging #594 (b2c2913) into main (e05e166) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##              main      #594   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files          305       306    +1     
  Lines        18013     18023   +10     
=========================================
+ Hits         18013     18023   +10     

Impacted Files	Coverage Δ
internal/config/config.go	100.00% <100.00%> (ø)
internal/database/sqlcommon/config.go	100.00% <100.00%> (ø)
internal/database/sqlcommon/sqlcommon.go	100.00% <100.00%> (ø)
internal/privatemessaging/groupmanager.go	100.00% <100.00%> (ø)
internal/privatemessaging/privatemessaging.go	100.00% <100.00%> (ø)
internal/privatemessaging/recipients.go	100.00% <100.00%> (ø)
pkg/fftypes/transaction.go	100.00% <100.00%> (ø)

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[awrichar]

Approved, although it will need to be merged after comments on #593 are addressed. codecov also shows a potential coverage problem, but unsure if it's real.