This allows the oldest LocalAuthor to be used as the default.

All JVMs must support UTF-8 encoding.

We check for compatibility by specifying a schema version and minimum
schema version in the code, storing them in the DB, and checking whether
the DB is too old for the code or vice versa.

Dev task #52. Known but unverified identities are also supported, but
currently unused. These will be used in future for contacts who've been
introduced but not verified face to face.

The new format is simpler but less efficient for small integers, short
strings and short byte arrays.

Note that this is only the generator part of Fortuna, not the
accumulator. The generator requires a seed, which is provided by a
platform-specific implementation of SeedProvider. On Linux the
implementation reads the seed from /dev/urandom.

If the schema of the database is incompatible with the schema expected
by the code, the database throws a DbSchemaException. LifecycleManager
indicates the error to BriarService, which uses HomeScreenActivity to
show a notification and quit the app.

Offered and requested message IDs are stored in the database rather than
being owned by DuplexConnections. This paves the way for moving to a
simplex transport layer.

The distinction between inbox groups and other groups can be maintained
internally, there's no need to represent it on the wire.

Private messages are now the same as group messages, but groups can be
private or public. When a contact is added, a private group is created
and designated as the inbox for exchanging private messages with the
contact.

This produces a saner user experience when devices have differing
clocks.

The Request packet now contains a list of message IDs, rather than a
bitmap referring to the list of messages IDs in the Offer. This allows
the Request to be understood out of context, e.g. if the Offer and
Request are sent over separate connections or a connection is replayed.

The method used by Commons IO to get the available disk space fails on
Android devices that lack a df binary - use the Android API instead.

This removes the other projects' dependency on the Android API.