Simplify Connection.upgradeFrom()/upgradeTo()

[sbordet]

Jetty version
9.4.30

Description
Connection.upgradeFrom()/upgradeTo() has some wording in the javadocs regarding the ownership of the ByteBuffer and who is responsible to release it back to the pool.

However, this is often difficult to do because the upgrade code is invoked from places that cannot easily return the ByteBuffer to the pool, or must do so at a later time.

Likewise, upgradeTo() implementations may need to feed the ByteBuffer into already existing pooled buffers and are unsure of whether they own the ByteBuffer or not and whether they should return it to the pool.

The proposal is to make sure that all implementation of upgradeFrom() copy the remaining bytes into a "floating" ByteBuffer that does not come from the pool and hence does not need to be returned to it.

In this way, upgradeTo() implementations are free to use this "floating" ByteBuffer as they like: parse it immediately, or store it away to parse it later, or copy it into another pooled buffer, etc.

Care should be taken to make sure that the original network buffer is correctly released back to the pool after the remaining bytes have been copied into the "floating" ByteBuffer.

[sbordet]

After discussion during review, changing the current behavior in 9.4.x is a risk, as upgradeTo() implementation are supposed to pool the buffer they receive.
Pooling a "floating" buffer would be bad since it won't have the right capacity and may lead to BufferOverflowExceptions.

Moving this change to Jetty 10.