"Failed to initialize transaction" when removing a Netconf device

When an attempt to unmount a netconf device is made through restconf, sometimes the response is:

"error-tag":"operation-failed"
"error-message":"Error while deleting data"
"error-info":"java.lang.RuntimeException: RemoteDevice

: Failed to prepare candidate. Failed to initialize transaction ..."

This bug can be hit pretty reliably (over 90% hit rate) by mounting 200 devices (or more), querying each of them for data in a rapid succession (possibly using a multithreaded process) and then unmount them. The bug hits between devices number 100 and 140. An unfinished test suite doing this is available here: https://git.opendaylight.org/gerrit/#/c/28730.

Attached is a log file fragment from the time when one of these failures was observed. It is fairly large but the first exception seen there suggests that the code handling the connection to netconf somehow lost track of the netconf message IDs and choked. This might be also restconf problem but it occurs in netconf and thus it is reported as netconf bug.