[Connection] Optimize flush

[abhinavsingh]

References

• [Core] Default send buffer size must be configurable #980 (comment)
• [Core] Default send buffer size must be configurable #980 (comment)

as observed by @softhub-software-development

TL;DR:

1. Avoid .tobytes() conversion
2. Avoid using len(mv)
3. Assemble multiple mv if we can batch them in a single send