[filesystem] restructured SFTP code

moved implementation of SFTPSession into own class file with header and
defined a SFTP.h with required forward declarations to reduce build
overhead.
[filesystem] fix: changed include to SFTPSession.h
[filesystem] splitted SFTPSessionManager
SFTPFile: Use queue for parallel queries
BufferQueue: fixed compilation bugs
debugging in progress
declare sftp to be an internet stream
SFTPFile: use own session for reading

If parallel read is used in combination with anything else libssh crashes.
To prevent this every file opens a new connection to the server which is
used only for reading.
SFTPFile: propper EOF handling and fixed memleak

previously only the request handles were removed leaving the request
and their allocated memory behind.