fix: verify if async task was concurrently deleted

[jyeshe]

Fix for:

testnet_1        | 2022-06-27T13:56:58.027177802Z 13:56:58.021 [error] gen_server 'Elixir.AeMdw.Sync.AsyncTasks.Producer' terminated with reason: no match of right hand value not_found in 'Elixir.AeMdw.Database':'fetch!'/2 line 128
testnet_1        | 2022-06-27T13:56:58.028565850Z 13:56:58.023 [error] CRASH REPORT Process 'Elixir.AeMdw.Sync.AsyncTasks.Producer' with 0 neighbours crashed with reason: no match of right hand value not_found in 'Elixir.AeMdw.Database':'fetch!'/2 line 128
testnet_1        | 2022-06-27T13:56:58.030062568Z 13:56:58.024 [error] Supervisor 'Elixir.AeMdw.Sync.AsyncTasks.Supervisor' had child 'Elixir.AeMdw.Sync.AsyncTasks.Producer' started with 'Elixir.AeMdw.Sync.AsyncTasks.Producer':start_link([]) at <0.2606.0> exit with reason no match of right hand value not_found in 'Elixir.AeMdw.Database':'fetch!'/2 line 128 in context child_terminated
testnet_1        | 2022-06-27T13:56:58.031508276Z 13:56:58.024 [error] gen_server <0.2609.0> terminated with reason: {{{badmatch,not_found},[{'Elixir.AeMdw.Database','fetch!',2,[{file,"lib/ae_mdw/database.ex"},{line,128}]},{'Elixir.Enum','-map/2-lists^map/1-0-',2,[{file,"lib/enum.ex"},{line,1396}]},{'Elixir.AeMdw.Sync.AsyncTasks.Store',fetch_unprocessed,1,[{file,"lib/ae_mdw/sync/async_tasks/store.ex"},{line,32}]},{'Elixir.AeMdw.Sync.AsyncTasks.Producer',next_state,1,[{file,"lib/ae_mdw/sync/async_tasks/producer.ex"},{line,107}]},{'Elixir.AeMdw.Sync.AsyncTasks.Producer',handle_call,3,[{file,"lib/ae_mdw/syn..."},...]},...]},...} in 'Elixir.GenServer':call/3 line 1023