fixes for bug 2870

[Wesley Craig]

I've attached fixes for bug #2870, that xfer can cause sync_client to  
die:

	https://bugzilla.andrew.cmu.edu/show_bug.cgi?id=2870

I also discovered that rename has a similar problem, and included a  
fix for that as well.  As I was analyzing this problem, I came to the  
conclusion that the "allow a certain number of errors" code in  
sync_client is serious flawed.  Under some circumstances, it will  
allow the replica to become out of sync with the primary backend.   
While it will log these fatal errors, they are mixed in with very  
similar non-fatal errors.  And more importantly, while sync_client  
usually dies when it encounters a fatal error, under some  
circumstances it will merely log an error and keep running.

:wes