Extract TimeoutFilter to a seperate file.
Add NumberOfRequestsPerSecond.

NumberOfRequestsPerSecond provides a counter which represents the number of
requests per second.

Internally, it has two counters for efficiency: front count and back count.
getNumberOfRequestsPerSecond method returns only the front count.
increase method does not increase the front count directly. Instead, it
increases the back count. Every second, the front count is updated with the
value of the back count and the back count is initialized back to 0.
Register MBeans on server start.

Currently, a sigar mbean is registered when SystemMetricsMonitor starts.
Registering mbean each time an actor restarts seems to be not a good idea,
because mbean server has the same life cycle with JVM. This patch makes
Beyond server register sigar mbean on start.

This patch also makes server register NumberOfRequestsPerSecondMBean.
Extract JMXConnector creation as JMXConnectorMixin trait.

Currently, SystemMetricsMonitor creates a JMXConnector on preStart. This patch
extracts JMXConnector creation as JMXConnectorMixin. After this patch, a class
which wants to use JMXConnector can use the member variable named jmxConnector
with JMXConnectorMixin.
Add RequestCounter.

RequestCounter is an actor which has a MBean proxy of
NumberOfRequestsPerSecond. When this actor receives an Increment message, it
calls increment method of the proxy.
Add RequestsCountFilter.

RequestsCountFilter is a global filter. It must be the first in WithFilters to
count all requests because later filters might not be applied.
Refactoring SystemMetricsMonitor.

Currently, MBeans are initialized on preStart and create Receive with the
MBeans. But there is no need to do it. MBeans can be initialized in the
constructor. This makes receiver simpler because receiver can use MBeans as
a member variable.
Make SystemMetricsSupervisor monitor the number of reqests.

Currently, SystemMetricsSupervisor monitors system loads and memory usages, and
writes them to database every 10 seconds. This patch makes
SystemMetricsSupervisor also monitor the number of requests per second.