Requests queue is not cleared in case of reject-nym transactions.

Build version:
indy-node: 1.6.601
indy-plenum: 1.6.539
Test description: 1 machine sends 5 nyms/sec to the pool by a user which has no role in the ledger.
Preconditions: Add a DID without role to the ledger (000000000000000000000000Trustee3)
Steps to Reproduce:
1. Run perf_processes.py from 1 AWS agent with following parameters:

python3.5 perf_processes.py -g pool_transactions_genesis -m t -n 1 -c 200 -l 5 -y one -k nym -s=000000000000000000000000Trustee3 --load_time=3600 -b=180 -d=~/Documents/logs -o=~/Documents/logs/load_script.csv

Expected results: 18k rejects in logs
Actual results: Test was stopped after 35-40 minutes, there were 9795 rejected nyms, 100 failed and 23 nacked because of a View Change.
Additional info:
Logs: evernymr33 ~/logs/indy-1593/
Priority justification:
When the test is executed and there is no additional load afterwards, the requests stay in queue and can not be released so it increases the load on nodes and may lead to memory overflow (DDOS attack from users which are registered in ledger and don't have enough privileges to write transactions/don't have enough tokens for fees)