Don't try to create allocations while starting up SenderAccountManager

[gusinacio]

Our system assumes that indexer service is capable of checking if an allocation exists or not and accepting receipts and then try to create a SenderAllocation if it doesn't exist.

But while starting up, SenderAllocations aren't not up yet but we are already receiving receipts. We should check if SenderAccountManager is starting up before trying to create a SenderAllocation.