so a new allocation is not needed in MarshalBinary and sendPacket.

Here are some profiling results while downloading a file (file size is about 1GB),

before this patch:

1254.24MB 55.18% 55.18%  1254.24MB 55.18%  github.com/pkg/sftp.sshFxpDataPacket.MarshalBinary
  991.81MB 43.63% 98.81%   991.81MB 43.63%  github.com/pkg/sftp.fileget
       1MB 0.044% 98.86%  1255.24MB 55.22%  github.com/pkg/sftp.(*packetManager).maybeSendPackets
    0.50MB 0.022% 98.88%  1260.24MB 55.44%  github.com/pkg/sftp.(*packetManager).controller
         0     0% 98.88%   991.81MB 43.63%  github.com/pkg/sftp.(*Request).call
         0     0% 98.88%   993.31MB 43.70%  github.com/pkg/sftp.(*RequestServer).Serve.func1.1
         0     0% 98.88%   993.31MB 43.70%  github.com/pkg/sftp.(*RequestServer).packetWorker
         0     0% 98.88%  1254.24MB 55.18%  github.com/pkg/sftp.(*conn).sendPacket
         0     0% 98.88%  1254.24MB 55.18%  github.com/pkg/sftp.sendPacket

with this patch:

1209.48MB 98.46% 98.46%  1209.48MB 98.46%  github.com/pkg/sftp.fileget
       2MB  0.16% 98.63%     7.50MB  0.61%  github.com/pkg/sftp.recvPacket
         0     0% 98.63%        8MB  0.65%  github.com/drakkan/sftpgo/sftpd.Configuration.handleSftpConnection
         0     0% 98.63%  1209.48MB 98.46%  github.com/pkg/sftp.(*Request).call
         0     0% 98.63%        8MB  0.65%  github.com/pkg/sftp.(*RequestServer).Serve
         0     0% 98.63%  1209.98MB 98.50%  github.com/pkg/sftp.(*RequestServer).Serve.func1.1
         0     0% 98.63%  1209.98MB 98.50%  github.com/pkg/sftp.(*RequestServer).packetWorker
         0     0% 98.63%     7.50MB  0.61%  github.com/pkg/sftp.(*conn).recvPacket (inline)

This way we can use the same method in both server and request-server

maxTxPacket was only used to get the size for the read packet it is not
needed anymore

…ation