Introduce adapter and handler sub-packages under web.server following a
review prompted by the addition of the session package and the package
cycle it brought in based on dependency on session.WebSessionManager.

This commit adds initial support for a maintaining a server-side
session with attributes across HTTP requests. The WebSession
abstraction can be accessed via WebServerExchange from a WebFilter or
the target WebHandler.

The session sub-package contains additional abstractions for creating
and managing sessions providing a basis for extensibility (e.g. Spring
Session). Those include WebSessionManager, SessionIdStrategy, and
SessionStore along with a cookie-based session id strategy and an
in-memory session store in use by default.

Note that the current API does not provide a way to invalidate or
re-create the session from server side code.

setComplete replaces writeHeaders as a more general lifecycle method
to perform any kind of handling at the end of request processing, for
example to ensure headers are written if not already.

beforeCommit provides an extension point for an action to be invoked
just before the response is committed, e.g. adding headers/cookies.

We use it only for Reactor Net support now.

Byte buffer abstraction

Added DataBuffer and DataBufferAllocator, and provided a default NIO
ByteBuffer-based implementation of those, as well as a Netty
ByteBuf-based version.

-> reactor.Mono : reactor.core.publisher.Mono
-> reactor.Flux : reactor.core.publisher.Flux
-> reactor.Processors : reactor.core.publisher.Processors
-> reactor.Timers : reactor.core.timer.Timers
-> reactor.Subscribers : reactor.core.subscriber.Subscribers