make stress when dble is starting, server resource is exhaust, and client hangs

[actiontech-bot]

Raised by: @FlyingMao Steps:

1. Prepare a vm with 4-core
2. Start dble
3. Sysbench 100-threads test with following lua:

function event(thread_id)
local table_name
local a=sb_rand(1, oltp_table_size-1)
local b=a+1
table_name = "sbtest".. sb_rand_uniform(1, oltp_tables_count)
rs = db_query("SELECT max(pad) from " .. table_name .. " where id between " .. a .. " and " ..b)
end

4. Found:
   1. dble use up to 400% cpu
   2. sysbench hang