WL#1265: Fix proper ALTER/DROP support in the SP cache.

New sp_cache C API. When an SP is dropped, old caches (in other threads)
become invalid and are cleared.
Also, the caches in THD are only created on demand.


Docs/sp-imp-spec.txt:
  Brough the SP cache docs up-to-date.
sql/mysqld.cc:
  Initialize SP cache.
sql/sp.cc:
  New C API for SP cache.
sql/sp_cache.cc:
  New C API for sp_cache.
  The class sp_cache is still used, but not directly. The C functions makes takes
  care of updating caches when SPs are dropped. (This is done in the simplest
  possible way, by simply detecting drops and then clear all old caches.)
  The API is also designed so that the sp_cache is created on demand.
sql/sp_cache.h:
  New C API for sp_cache.
  The class sp_cache is still used, but not directly. The C functions makes takes
  care of updating caches when SPs are dropped.
  The API is also designed so that the sp_cache is created on demand.
sql/sql_class.cc:
  The new sp_cache API creates the caches on demand, to avoid allocating it
  when it's not needed.

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