Analysis:
When thread cache is enabled, it does not properly initialize
thd->start_utime when a thread is picked from the thread cache.
This breaks the quota management mechanism. 
THD::time_out_user_resource_limits() resets 
m_user_connect->conn_per_hour to 0 based on thd->start_utime

Fix:
Initialize start_utime when cached thread is reused.

Notes:
Enabled back tests which were disabled because of this issue.