Tuning payload timeout

[YoshihitoAso]

close #78

I made a modification to improve block commit throughput by reducing the PayloadTimeout value.

• 800 ms -> 200 ms

I tested that block generation did not stop even when the network was loaded with 10,000 transactions containing 2000 byte strings. Please refer to the PR below for detail test settings.

• Add load test modules ibet-Network#162

Test results

• 200msec_2000_10000_general.log (non-validator, Tx source)
• 200msec_2000_10000_validator.log

[purplesmoke05]

LGTM