Allow slow migrations

[DifferentialOrange]

Module forbids to run migrations which are longer than one hour per storage

migrations/migrator/init.lua

Line 73 in 9b06e63

local applied_migrations, err = conn:call('__cluster_rpc_call_local', { 'migrator', 'upgrade' }, {timeout = 3600})

The value is hardcoded and cannot be changed. We should make it configurable, if possible, or at least make the threshold bigger, since

migrations/migrator/init.lua

Line 72 in 9b06e63

-- migrations might take long time, so net.box timeout should be disabled

it seems that the meaning of timeout = 3600 was supposed to be "timeout is infinity".