XDCR throughput drop after adding scram-sha support

XDCR rate degraded from 160K to 120K items/sec 
(all results: http://showfast.sc.couchbase.com/#/timeline/Linux/xdcr/init_single/all)

XDCR configuration: 
1 to 1 replication
sourceNozzlePerNode = 2
targetNozzlePerNode = 4

Detected on build 5.5.0-2167

goxdcr changes in 2267: https://github.com/couchbase/goxdcr/commit/1a384835487379c32e6714aa7e4097253534b35b

5.5.0-2166 and 5.5.0-2167 builds comparison:
source: [http://cbmonitor.sc.couchbase.com/reports/html/?snapshot=titan_c1_550-2166_init_xdcr_ebce&snapshot=titan_c1_550-2167_init_xdcr_e4dd
d|http://cbmonitor.sc.couchbase.com/reports/html/?snapshot=titan_c1_550-2166_init_xdcr_ebce&snapshot=titan_c1_550-2167_init_xdcr_e4dd]estination: http://cbmonitor.sc.couchbase.com/reports/html/?snapshot=titan_c2_550-2166_init_xdcr_7973&snapshot=titan_c2_550-2167_init_xdcr_96a2

Logs from 5.5.0-2167 run:

source: https://s3-us-west-2.amazonaws.com/perf-artifacts/jenkins-titan-2836/172.23.96.105.zip
destination: https://s3-us-west-2.amazonaws.com/perf-artifacts/jenkins-titan-2836/172.23.96.100.zip