BUG-1886: eliminate sychronized blocks

The tree state is guarded by the read-write lock, so there is no need
for the additional, overly large, critical section. This allows empty
operations to complete completely lockfree.

Change-Id: Iee01f4745a4927e6dc5093252c15d7d4e3959e9a
Signed-off-by: Robert Varga <rovarga@cisco.com>