Sync, Async mappings and Middleware features

[RomanEmreis]

Introduced new features:

• async
• sync
• middleware

The default and full are now include:

# Default HTTP/1 only server with async endpoints mapping
default = ["http1", "async", "middleware"]
# HTTP/1 and HTTP/2 server with sync and async endpoints mappings
full = ["http1", "http2", "async", "sync", "middleware"]

Introduced new minimal API features:

• mini
• mini2

# Mimimal HTTP/1
mini = ["http1", "async"]
# Mimimal HTTP/2
mini2 = ["http2", "async"]