Don't attempt to send transactions if Dendrite is shutting down

[S7evinK]

This should avoid confusions with logs like:

time="2024-04-08T08:38:45.104235081Z" level=error msg="Failed to set \"scs.ems.host\" as assumed offline" func="github.com/matrix-org/dendrite/federationapi/statistics.(*ServerStatistics).Failure" file="github.com/matrix-org/dendrite/federationapi/statistics/statistics.go:204" error="sqlutil.WithTransaction.Begin: sql: database is closed"
time="2024-04-08T08:38:45.104239201Z" level=error msg="Failed to set \"obermui.de\" as assumed offline" func="github.com/matrix-org/dendrite/federationapi/statistics.(*ServerStatistics).Failure" file="github.com/matrix-org/dendrite/federationapi/statistics/statistics.go:204" error="sqlutil.WithTransaction.Begin: sql: database is closed"

or

time="2024-04-08T08:38:45.105235411Z" level=error msg="Failed to get pending EDUs for \"retro76.net\"" func="github.com/matrix-org/dendrite/federationapi/queue.(*destinationQueue).getPendingFromDatabase" file="github.com/matrix-org/dendritefederationapi/queue/destinationqueue.go:258" error="sqlutil.WithTransaction.Begin: sql: database is closed"

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 67.40%. Comparing base (8aa088f) to head (43ac98d).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3356      +/-   ##
==========================================
+ Coverage   67.36%   67.40%   +0.03%     
==========================================
  Files         521      521              
  Lines       47364    47366       +2     
==========================================
+ Hits        31905    31925      +20     
+ Misses      11450    11439      -11     
+ Partials     4009     4002       -7     

Flag	Coverage Δ
unittests	52.74% <100.00%> (+0.04%)	⬆️

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[devonh]

Seems sane to me.