Introduce a way to update broker service configuration dynamically

[rdhabalia]

As discussed at #176

We need to think well how to expose the whole thing in consistent way. You have the values from the config file + the values in ZK. There should be a clear way to understand what is the actual configuration that is:
Supposed to be applied to all brokers
It's actually being applied in a given broker...
Other problem is how to clearly identify which values are dynamically tunable (eg: zk servers string will not be dynamic...)

@merlimat @saandrews : I have created very initial level commit to propose a possible approach for this feature. Can you please provide your thoughts or ideas on it. If it makes sense then we can move forward on this approach.

[merlimat]

Few random ideas (haven't looked at the code yet):

• Mark dynamic settings with an annotation in ServiceConfiguration
• Have a way to register for config changes events (this may be useful for certain class of variables)
• Otherwise, just read the "dirty" value from the ServiceConfiguration instance (don't store it in local variable and don't use any mutex
• Add a REST handle to change key/value pairs, that only accept variables marked as dynamic
• Store all dynamically configured variables in a single z-node in "properties" format
• At startup log the content of that z-node and log each time a change is detected

[rdhabalia]

• Mark dynamic settings with an annotation in ServiceConfiguration : [Yes, Included in commit]
• Have a way to register for config changes events (this may be useful for certain class of variables) [Yes, Included in commit]
• Otherwise, just read the "dirty" value from the ServiceConfiguration instance (don't store it in local variable and don't use any mutex [Have done similar change]
• Add a REST handle to change key/value pairs, that only accept variables marked as dynamic [Yes, Included in commit]
• Store all dynamically configured variables in a single z-node in "properties" format [Yes, Included in commit]
• At startup log the content of that z-node and log each time a change is detected [Yes, Will do]

This commit includes almost all above suggestions. Once you review and if you think it is fine then I will go ahead and complete full changes.