add changelog, version bump to v0.1.8

add changelog, version bump to v0.1.7

add changelog, version bump to v0.1.6

add changelog, version bump to v0.1.5

add changelog, version bump to v0.1.4

add changelog, version bump to v0.1.3

better replicator with customized worker process

don not delete pending_x attributes for session

if the session was commited multiple times, it crashes.