include/cpp-btree: adapt from abseil::btree

include/cpp-btree: adapt from abseil::btree

adapt based on btree implementation from Abseil Common Libraries.

with following changes:

* remove abseil dependencies. as abseil libraries do support C++11,
  they are written in C++11 compatible way. and use its homebrew
  alternatives for the facilities offered by C++14 and C++17. but
  Ceph is in C++17, so replace those facilities using the ones
  from the C++ standard library
* change `key_compare_to_adapter` to adapt to arithmetic types, and
  those who have `compare()` defined.
* move SlotPolicy into *_params types, simpler this way.
* continue using *_fields types. the layout facilities is nice. but
  we don't need this flexibility at this moment.
* continue using `empty_base_handle` for EBCO, CompressedTuple is
  nice, but it involves more dependencies which we don't want at
  this moment.
* node handle support is dropped. it's part of C++17 standard for
  node-based containers. we don't use it at this moment. but it's
  relatively simple if we want to add it back.

Signed-off-by: Kefu Chai <kchai@redhat.com>