made PME load balancing + DD DLB more efficient

The DD dynamic load balancing is now limited, such that the fastest
timed PME load balancing cut-off setting can always be used.
Fixes #1089

Change-Id: I3216dfd5a8b2b0676eee5519e08cf36e06047251
fixed nbnxn hybrid mode with OpenMP

Fixes #1100

Change-Id: I2f3c95f698852ab496a648a04555c0e9ecb57463