Adjust maxPeriod from 30 to 120 days for Bitfinex according to Bitfinex

[vansonlin]

Adjust maxPeriod from 30 to 120 days for Bitfinex according to Bitfinex

Closes #651

Link to Bitfinex's Announcement: https://www.bitfinex.com/posts/557

TESTING STAGE

Successfully tested with my bot

Types of changes

• [x] Bug fix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• [x] I have read CONTRIBUTING.md
• [x] I fully understand Github Flow.
• [x] My code adheres to the code style of this project.
• [x] I have updated the documentation in /docs if I have changed the config, arguments, logic in how the bot works, or anything that understandably needs a documentation change.
• [x] I have updated the config file accordingly if my change requires a new configuration setting or changes an existing one.
• [x] I have tested the bot with no issues for 24 continuous hours. If issues were experienced, they have been patched and tested again.

[vansonlin]

My current strategy has max period as 90 days and it did place a few 90-day orders. But I do see few 120 days orders matched, so this definitely works.

[Evanito]

Very well then, no need to delay. Thank you for the contribution!