move log, ondisklog, missing from PG to PGLog

PG::log, PG::ondisklog, PG::missing are moved from PG to a new PGLog
class and are made protected data members. It is a preliminary step
before writing unit tests to cover the methods that have side effects
on these data members and define a clean PGLog API. It improves
encapsulation and does not change any of the logic already in
place.

Possible issues :

* an additional reference (PG->PGLog->IndexedLog instead of
  PG->IndexedLog for instance) is introduced : is it optimized ?

* pg_log_t::splice converts its const_iterator into an iterator using
  std::advance which goes thru each element of the logs up to the
  position of the const_iterator. If the log is long and the call is
  too frequent, there will be a noticeable performance penalty. Is it
  called frequently enough to be a problem ?

* rewriting log.log into pg_log.get_log().log affects the readability
  but should be optimized and have no impact on performances

The guidelines followed for this patch are:

* const access to the data members are preserved, no attempt is made
  to define accessors

* all non const methods are in PGLog, no access to non const methods of
  PGLog::log, PGLog::logondisk and PGLog::missing are provided

* when methods are moved from PG to PGLog the change to their
  implementation is restricted to the minimum.

* the PG::OndiskLog and PG::IndexedLog sub classes are moved
  to PGLog sub classes unmodified and remain public

A const version of the pg_log_t::find_entry method was added.

A const accessor is provided for PGLog::get_log, PGLog::get_missing,
PGLog::get_ondisklog but no non-const accessor.

Arguments are added to most of the methods moved from PG to PGLog so
that they can get access to PG data members such as info or log_oid.
A bound method pointer to PG::remove_snap_mapped_object is provided to
the merge_log, rewind_divergent_log and merge_old_entry. These additions
are not repeated in the method notes below.

The PGLog method are sorted according to the data member they modify.

//////////////////// missing ////////////////////

* The pg_missing_t::{got,have,need,add,rm} methods are wrapped as
  PGLog::missing_{got,have,need,add,rm}

//////////////////// log ////////////////////

* PGLog::get_tail, PGLog::get_head getters are created

* PGLog::set_tail, PGLog::set_head, PGLog::set_last_requested setters are created

* PGLog::index, PGLog::unindex, PGLog::add wrappers, PGLog::reset_recovery_pointers are created

* PGLog::splice is a wrapper for pg_log_t::splice that converts its
  const_iterator into an iterator

* PGLog::clear_info_log replaces PG::clear_info_log

* PGLog::trim replaces PG::trim

//////////////////// log & missing ////////////////////

* PGLog::claim_log is created with code extracted from
  PG::RecoveryState::Stray::react.

* PGLog::split_into is created with code extracted from
  PG::split_into.

* PGLog::recover_got is created with code extracted from
  ReplicatedPG::recover_got.

* PGLog::activate_not_complete is created with code extracted
  from PG::active

* PGLog:proc_replica_log is created with code extracted from
  PG::proc_replica_log

* PGLog:write_log is created with code extracted from
  PG::write_log

* PGLog::merge_old_entry replaces PG::merge_old_entry

* PGLog::rewind_divergent_log replaces PG::rewind_divergent_log

* PGLog::merge_log replaces PG::merge_log

* PGLog:write_log is created with code extracted from PG::write_log. A
  non-static version is created for convenience but is a simple
  wrapper.

* PGLog:read_log replaces PG::read_log. A non-static version is
  created for convenience but is a simple wrapper.

* PGLog:read_log_old replaces PG::read_log_old.

Signed-off-by: Loic Dachary <loic@dachary.org>