Plasma : Indexer killed by OOM killer when index key size is 30K

While testing a toy build with the potential fix for CBSE-5465, observed an OOM killing for the indexer process.

Server logs :
https://s3-us-west-2.amazonaws.com/perf-artifacts/jenkins-themis-dev-512/172.23.96.16.zip
https://s3-us-west-2.amazonaws.com/perf-artifacts/jenkins-themis-dev-512/172.23.96.17.zip
https://s3-us-west-2.amazonaws.com/perf-artifacts/jenkins-themis-dev-512/172.23.96.20.zip
https://s3-us-west-2.amazonaws.com/perf-artifacts/jenkins-themis-dev-512/172.23.96.23.zip
https://s3-us-west-2.amazonaws.com/perf-artifacts/jenkins-themis-dev-512/172.23.97.177.zip

cbmonitor graphs :
http://cbmonitor.sc.couchbase.com/reports/html/?snapshot=themis_550-10000_build_incrindex_1528#7711a4312344385553a6ea998067b44a
http://cbmonitor.sc.couchbase.com/reports/html/?snapshot=themis_550-10000_build_incrindex_1528#cc50daefc1591f1182e95aa84eaddf36

Toy build : http://server.jenkins.couchbase.com/job/toy-unix/2879/artifact/couchbase-server-enterprise-5.5.0-10000-centos7.x86_64.rpm