after processing a packet we keep in memory the allocated slices and we reuse
them for new packets.
Slices are allocated in:

- recvPacket
- when we receive a sshFxpReadPacket (downloads)

The allocated slices have a fixed size = maxMsgLength.

Allocated slices are referenced to the request order id and are marked for reuse
after a request is served in maybeSendPackets.

The allocator is added to the packetManager struct and it is cleaned at the end
of the Serve() function.

This allocation mode is optional and disabled by default

Other minor changes as per review comments

we can keep compatibility removing the error return value from
RequestServerOption

we need a POSIX path filepath.IsAbs can give unexpected results on Windows