Introduce GIT_OPT_SET_ODB_LOOSE_PRIORITY and GIT_OPT_SET_ODB_PACKED_PRIORITY
to allow overriding the default priority values for the default ODB
backends. Libgit2 has historically assumed that most objects for long-
running operations will be packed, therefore GIT_LOOSE_PRIORITY is
set to 1 by default, and GIT_PACKED_PRIORITY to 2.
When a client allows libgit2 to set the default backends, they can
specify an override for the two priority values in order to change
the order in which each ODB backend is accessed.

The majority of functions are named `from_something` (with an
underscore) instead of `fromsomething`.  Update the blob functions for
consistency with the rest of the library.

This change frees a copy of a cached object in odb_otype_fast().

Move to the `git_error` name in the internal API for error-related
functions.

In include/git2/odb.h it states that callback can also return
positive value which should break looping.

Implementations of git_odb_foreach() and pack_backend__foreach()
did not respect that.

Use the new object_type enumeration names within the codebase.

In commit 7ec7aa4a7 (odb: assert on logic errors when writing objects,
2018-02-01), the check for whether we are trying to overflowing the fake
stream buffer was changed from returning an error to raising an assert.
The conversion forgot though that the logic around `assert`s are
basically inverted. Previously, if the statement

    stream->written + len > steram->size

evaluated to true, we would return a `-1`. Now we are asserting that
this statement is true, and in case it is not we will raise an error. So
the conversion to the `assert` in fact changed the behaviour to the
complete opposite intention.

Fix the assert by inverting its condition again and add a regression
test.