merge/dq-txes: calls which builds auth sample dont respond while supernode "initializing"

[mbg033]

e.g.:

• /debug/auth_sample/
• /dapi/v2.0/presale/

2019-09-03 17:27:35.681     7f2456acf700        DEBUG   supernode.fullsupernodelist     src/rta/fullsupernodelist.cpp:381       building auth sample for height 394579 (blockchain_based_list_height=394579) and PaymentID '23312'
supernode: /home/mbg033/dev/graft-project/graft-ng/modules/cryptonode/src/utils/sample_generator.cpp:53: void graft::generator::seed_uniform_select(const string&): Assertion `!str.empty() && str.size()%sizeof(uint32_t) == 0' failed.

UPD:
call

ctx.global.get(CONTEXT_KEY_FULLSUPERNODELIST, ...)

could lock for up to ~25 seconds