Go-XDCR: Data loss in destination bucket is left undetected (pre-replication doesn't happen during idle-xdcr)

Build
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3.5.0-1270

Testcase
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./testrunner -i INI_FILE.ini -p enable_goxdcr=True,checkpoint_interval=60,replication_type=xmem -t xdcr.uniXDCR.unidirectional.test_idle_xdcr_dest_flush,rdirection=unidirection,topology=chain,replication_type=xmem,items=10000

Steps
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1. C1 [.170,.171] --> C2 [.120,.169]
2. Add 1000 keys to C1
3. Flush target bucket
4. Make sure pre_replicates happen regularly even in idle xdcr to discover target is missing updates. In erlang xdcr, this pre-replication happens every 60s.

[2015-02-17 18:37:17,403] - [task:487] WARNING - Not Ready: vb_active_curr_items 0 == 10000 expected on '172.23.107.120:8091''172.23.107.169:8091', default bucket
[2015-02-17 18:37:22,443] - [task:487] WARNING - Not Ready: curr_items 0 == 10000 expected on '172.23.107.120:8091''172.23.107.169:8091', default bucket

Target bucket emptiness is not detected until 12 mins (Of course, the test fails soon after

[2015-02-17 18:49:58,762] - [task:487] WARNING - Not Ready: vb_active_curr_items 0 == 10000 expected on '172.23.107.120:8091''172.23.107.169:8091', default bucket