Add min price for execution

[antho1404]

Right now an execution can be created with 0 tokens.

I added the support of a minimum that is set for now to 10000atto. This parameter is part of the module and present in the genesis.
I also added a config to have a default price from the clients (api/orchestrator)

The good thing of having this parameter in the genesis is that we could do some vote to change this limit and we can play with proposal/votes.

[krhubert]

Why this should be set globally? I think that value should be set by service creator (or even voting by service creator and runners). What do you think?

[antho1404]

Totally agree that the service creator/runner should set the price, this is just a min value that can later be set to 0 if necessary but the main goal here is to be able to actually see this economy. For now, everything is ready for it but we still accept free executions.

[NicolasMahe]

did you test to change the parameter on a running blockchain?

[NicolasMahe]

what about also putting the var runnerShare = sdk.NewDecWithPrec(9, 1) in params?

[antho1404]

what about also putting the var runnerShare = sdk.NewDecWithPrec(9, 1) in params?

Didn't want to change this on this PR but yes definitely something that we could add later.

[NicolasMahe]

The errors are pretty bad:
from the cli:

➜  engine git:(feature/default-exec-price) ✗ mesg-cli service:execute 9pifx2Hxq9sbJdhDRMKy2hevt9d7d2JbLiyJ6zsSSYEM task1 --data msg=Hello
 ›   Error: 2 UNKNOWN: transaction returned with invalid code 11

from the engine's logs

INFO[0140] Rejected bad transaction                      err="<nil>" module=mempool peerID= res="&{code:11 log:\"out of gas: out of gas in location: ReadFlat; gasWanted: 22261, gasUsed: 23058\" gas_wanted:22261 gas_used:23058 codespace:\"sdk\" }" tx=8D9DC7D6633960456FC0D71A3B131B5D68CC566574D2289D1E32F77F9A5EE4BC
ERRO[0140] finished unary call with code Unknown         error="transaction returned with invalid code 11" grpc.code=Unknown grpc.method=Create grpc.service=mesg.api.Execution grpc.start_time="2020-03-02T05:11:26Z" grpc.time_ms=2.777 module=grpc span.kind=server system=grpc

we really need to improve the returned error but not blocking for this PR