(ITS#9043) Improve loglevel sync logging

[ondra@openldap.org]

Full_Name: Ondrej Kuznik
Version: master
OS: 
URL: ftp://ftp.openldap.org/incoming/
Submission from: (NULL) (82.10.24.68)


The existing replication logs are quite hard to follow. In syncprov, messages
about different sessions mingle with no indication where they belong and many
important replication decisions are not logged at all. And what is logged is
very hard to follow and understand why the provider/consumer does what it does.

The following could be done to improve matters:
- prepend the request information to syncprov messages
- add information to existing messages, especially human-readable commentary
that can be followed as a story, giving context in terms of "X happened so doing
Y"
- log when checkpoints happen, what cookies are generated and why, ...

Most of the above is drafted here:
https://github.com/mistotebe/openldap/tree/its9043