txsource: delegation workload

[ptrus]

Part of: #2506

Delegation workload continuously makes escrow and reclaim escrow transactions between accounts. (note: the escrow balance is only funded at start which effectively means reclaim is also tested as a part of this workload).

Also fixes a bug in delegation queries (extracted into #2756)

TODO:

• probably should add unit tests for the delegation queries

[codecov]

Codecov Report

Merging #2752 into master will decrease coverage by 0.45%.
The diff coverage is 64.74%.

@@            Coverage Diff             @@
##           master    #2752      +/-   ##
==========================================
- Coverage   62.74%   62.29%   -0.46%     
==========================================
  Files         384      385       +1     
  Lines       36257    36412     +155     
==========================================
- Hits        22749    22682      -67     
- Misses      10617    10827     +210     
- Partials     2891     2903      +12     

Impacted Files	Coverage Δ
...o/consensus/tendermint/apps/staking/state/state.go	66.66% <ø> (+4.49%)	⬆️
...s-node/cmd/debug/txsource/workload/registration.go	0.00% <0.00%> (-74.44%)	⬇️
...oasis-node/cmd/debug/txsource/workload/workload.go	77.77% <ø> (ø)
...sis-node/cmd/debug/txsource/workload/delegation.go	65.16% <65.16%> (ø)
go/storage/mkvs/urkel/writelog/iterator.go	76.36% <0.00%> (-16.37%)	⬇️
...n/crypto/signature/signers/memory/memory_signer.go	71.42% <0.00%> (-4.77%)	⬇️
go/worker/storage/service_external.go	47.31% <0.00%> (-4.31%)	⬇️
go/worker/storage/committee/node.go	76.71% <0.00%> (-4.24%)	⬇️
go/runtime/client/watcher.go	73.84% <0.00%> (-2.31%)	⬇️
... and 24 more

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[ptrus]

thanks for review!