Cleanup airconnect.log log file during startup

[eizedev]

Describe the bug

See: #79 (comment)

The log file grows and will never be deleted. Check it during startup, and clean it up.

To Reproduce

Steps to reproduce the behavior.

Expected behavior

A clear and concise description of what you expected to happen.

Logfiles

If applicable, add logfiles to help understand your problem.

Hardware

Device on which you are running AirConnect-Synology):

• Synology NAS or Router model name: [e.g. ds415+]
• (optional) DSM Version [e.g. DSM 6.2.3-25426 Update 3]

Package

Which package from the releases page have you downloaded):

• package name: [e.g. AirConnect-x86-64-0.2.43.0-20201229.spk or the url]

Additional context

Add any other context about the problem here.