run_electrum_server does not verify open files limit is high enough

[shsmith]

Suggest adding logic like this near the top of run_electrum_server:

# prevent database corruption due to default open file limits
import resource
softlimit, hardlimit = resource.getrlimit(resource.RLIMIT_OFILE)
if hardlimit < 64000:
    print "Your open files limit is too low." 
    sys.exit()