Bug 963 - OSGi error in Topology manager component - After exiting

Bug 963 - OSGi error in Topology manager component - After exiting
mininet of13 simulation

* added check in onLinkRemoved() to check if link exists before
atempting to delete it (without check, lots of datastore transaction
errors were displayed on mininet shutdown because links were already
removed with nodes)
* added checks to check if node, connector or link exists before trying
to remove it (reads are cheap and delete exception can break the
transaction chain)
* made reads synchronous
* added illegalState exception handling to operation processor
* made affected links removal to be in the same transaction as
Node/NodeConnector removal(creating and applying snapshot for each link
is expensive)
* updated tests

Change-Id: I83b5f91569e64082ce13a4f379cd6261e63fbde6
Signed-off-by: Jan Hajnar <jhajnar@cisco.com>