syslog: fixes and updates to rsyslogd and logrotate.
Adjust lograte size to prevent a mismatch.
Change rsyslogd size to be actual size instead of a naive value.
Potentially fix bug #13699000 by increasing the maximum number of TCP sessions.

Rsyslog was rejecting agents from connecting because of MaxSessions, which caused those
agents to go into spirals of trying to restart their rsyslog worker over and over,
which then generated tons of DEBUG level logging on machine-0 because of the workers
restarting, which then spammed the log file a lot.
Merge branch 'master' into rsyslog-max-connections-1369900

Small conflict that can be trivially fixed.

Conflicts:
	utils/syslog/config.go
initial commit
added check and tests to stop add-units earlier if the destination machine doesn't exist, lp:1384732
extract relation membership caching into new type; not hoked up yet
extract relation membership caching into new type, which I remembered to add this time; not hooked up yet
figured out better interface
use errors annotate instead of fmt.Errorf
RelationContext now uses cache internally; uniter now hooked up and tested; context tests not yet updated
Update charm interface implementation.
fixed some tests, commented out others, will reinstate soon
add missing test case
Move code out of package_test.go.
Do not use MgoTestPackage in backups.
Expose backups.StoreArchive.
Set meta.Stored properly.
reinstate, expand factory tests to take caches into account
reinstate and slightly extend relevant tests; drop an irrelevant one
address review