Replace notion of 'validity' with that of 'attachment'

[kspangsege]

1. Rename Table::is_valid() -> Table::is_attached().
2. Modify Group::is_attached() so that in the case of a group managed by a shared group, it returns true when, and only when a transaction is active. This requires that it is no longer tied to the state of attachment of the slab allocator to an underlying file.

\cc @bmunkholm

[bmunkholm]

Ok to merge into master, since the rename only affects Java, and not customers as of now :-)
Going forward we should onle add the "&g_dyn_tabl_...." anchors to the documentation if it is reusable by he bindings - i.e. genericly described.

[kspangsege]

Brian, I don't think a flag based alternative would be any easier to read that the 'goto' version.

[kspangsege]

Alexander, I'll merge this pull request now, but feel free to review.