Fix persist reco packing for DataType 2018

cherry-picked from !1304 (merged) on 2018-patches branch

• [2018-05-10 00:11] Validation started with lhcb-lcg-dev4#520
• [2018-05-10 00:11] Validation started with lhcb-lcg-dev3#520
• [2018-05-10 00:12] Validation started with lhcb-clang-test#947
• [2018-05-10 00:13] Validation started with lhcb-gaudi-head#1862
• [2018-05-10 00:14] Validation started with lhcb-gaudi-head-py3#149
• [2018-05-10 00:15] Validation started with lhcb-tdr-test#146
• [2018-05-10 00:16] Validation started with lhcb-head#1848
• [2018-05-10 03:12] Validation started with lhcb-gaudi-head-py3#149
• [2018-05-10 03:15] Validation started with lhcb-head#1848
• [2018-05-10 03:26] Validation started with lhcb-tdr-test#146
• [2018-05-10 08:13] Validation started with lhcb-gaudi-merge#511

merged

mentioned in commit adfb1c3d