Merge branch 'release-v1.5.5'

Release v1.5.4 - global snapshot (iotaledger-archive#989)

* update snapshot file, milestone start index, and global snapshot time

* bump version

* update snapshot sig and previous epoch spend addresses file is now split

Merge branch 'release-v1.5.3'

Merge branch 'release-v1.5.2'

Merge branch 'release-v1.5.1'

Merge branch 'release-v1.5.0'

Merge pull request iotaledger-archive#735

change version

Reduce writes if no data changed.