* Add Proving object, which maintains contract id's to watch
* [proving] invoke callback when proof is required
# Conflicts:
# dagger/por/timing/periods.nim
# dagger/por/timing/prooftiming.nim
* [proving] check proof requirements for all our contracts
# Conflicts:
# tests/dagger/helpers/mockprooftiming.nim
* Update vendor/dagger-contracts
* [proving] call onProofRequired() when proof is required soon
* [proving] stop checking contracts that have ended
* [proving] Remove duplicated funcs
* [proving] Implement ProofTiming on top of smart contract
* [proving] Fix race condition in waitUntilNextPeriod()
Sometimes waitUntilNextPeriod would take a while to
determine the current period, leading to unexpected results.
Splits waitUntilNextPeriod() into getCurrentPeriod()
and waitUntilPeriod(), to ensure that we're really waiting
for the period that we think we're waiting for.
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