v0.30.3: bump version to 0.30.3, include libssh 0.9.3 (security fix) (#280)

! See #265 Regression for FTPES, FTPS on directory changes !

• Raise SFTP buffer size to 128 KiB to speed up I/O (see #90) (#275)
• update to libssh 0.9.3, see https://www.libssh.org/2019/12/10/libssh-0-9-3-and-libssh-0-8-8-security-release/
  containing also 0.9.2, see https://www.libssh.org/2019/11/07/libssh-0-9-2/