dpif-netdev: Garbage collect the exact match cache periodically.

On current master, the exact match cache entry can keep reference to
'struct dp_netdev_flow' even after the flow is removed from the flow
table.  This means the free of allocated memory of the flow is delayed
until the exact match cache entry is cleared or replaced.

If the allocated memory is ahead of chunks of freed memory on heap,
the delay will prevent the reclaim of those freed chunks, causing
falsely high memory utilization.

To fix the issue, this commit makes the owning thread conduct periodic
garbage collection on the exact match cache and clear dead entries.

Signed-off-by: Alex Wang <alexw@nicira.com>
Acked-by: Jarno Rajahalme <jrajahalme@nicira.com>

---
PATCH -> V2:
- Adopt Jarno's suggestion and conduct slow sweep to avoid introducing
  jitter.