Send pulses in parallel (#41772)

resources/migrations/001_update_migrations.yaml

src/metabase/api/alert.clj

src/metabase/api/common.clj

src/metabase/db/custom_migrations.clj

src/metabase/models/pulse.clj

src/metabase/models/pulse_channel.clj

src/metabase/task/send_pulses.clj

test/metabase/analytics/stats_test.clj

test/metabase/models/pulse_channel_test.clj

test/metabase/models/pulse_test.clj

test/metabase/task/send_pulses_test.clj

test/metabase/test/util.clj