Fix compilation with Swift 5.5.0 and 5.5.1

[dnadoba]

Motivation:

swift-nio does not compile with Swift 5.5.0 and Swift 5.5.1 as described in this issue: #2233
The underlying issue is described here swiftlang/swift#57346

Modifications:

• use @unchecked Sendable in Swift 5.5.0 and 5.5.1

Result:

swift-nio compiles again with Swift 5.5.0 and 5.5.1