relicense LGPL-2.1 code as LGPL-2.1 or LGPL-3.0

relicense LGPL-2.1 code as LGPL-2.1 or LGPL-3.0

The primary motivation to relicense is a desire to integrate with projects
that are licensed under the Apache License version 2.0.  Although opinions
vary, there are some who argue the the LGPL-2.1 and Apache-2.0 licenses
are not fully compatible.  We would like to avoid the ambiguity and
potential for controversy.

Projects we would like to consume that are Apache-2.0 licensed include
Seastar, OpenSSL (which is in the process of relicensing to Apache-2.0),
and Swagger (swagger.io).  Note that some of these are dynamically linked
or consumed via a high-level language and may or may not require a change
to LGPL-3.0, but providing the option for LGPL-3.0 certainly avoids any
uncertainty.

A few other source files are already incorporated into Ceph that claim an
Apache-2.0 license:

    src/common/deleter.h
    src/common/sstring.h
    src/include/cpp-btree

The Ceph developers would further like to provide a license option that is
more modern than the current LGPL-2.1.  LGPL-3.0 includes updated,
clarified language around several issues and is widely considered
more modern, superior license.

Signed-off-by: Sage Weil <sage@redhat.com>