Protect memroystore connection obejct with a mutex

[Sriep]

Wrapped the memorystore.connection map in an object and protected get and set methods with a mutex.
Addresses issue #265.

[platsko]

Not sure it is good practice to use a pointer to mutex: https://github.com/uber-go/guide/blob/master/style.md#zero-value-mutexes-are-valid

[Sriep]

The problem is lint raises an objection to copying mutex. Below are the lint errors if I change the mutex to not be a pointer:

core/memorystore/connection.go:120:45: copylocks: call of context.WithValue copies lock value: 0chain.net/core/memorystore.connections contains sync.RWMutex (govet)
		return context.WithValue(ctx, CONNECTION, cMap)
		                                          ^
core/memorystore/connection.go:169:45: copylocks: call of context.WithValue copies lock value: 0chain.net/core/memorystore.connections contains sync.RWMutex (govet)
		return context.WithValue(ctx, CONNECTION, cMap)
		                                          ^
core/memorystore/connection.go:211:10: copylocks: assignment copies lock value to cMap: 0chain.net/core/memorystore.connections contains sync.RWMutex (govet)
	cMap := c.(connections)
	        ^