Improved Bindings.getBinding(Class) caching for Datatype.class

Improved Bindings.getBinding(Class) caching for Datatype.class

Previously Datatype.class binding was never cached nor statically
constructed and made available as Bindings.DATATYPE which is only
sensible because Datatype class is a central building block of Databoard
and therefore its binding should also be readily available instead of
all clients having to request for it separately.

A weak hashmap cache was also added for signature strings in
BindingRequest because it was noticed that in conditions where code
makes a lot of requests for uncached binding, the cost of String
construction in BindingRequest.getSignature starts to dominate
cpu/memory costs. Two main problems in there were: constantly creating
new strings and using the very inefficient String.replaceAll instead of
String.replace(char, char).

gitlab #82

Change-Id: I50fbbc2bbe1e5d6b8b5864c3aed4c228022e66dc