MWL#68 Subquery optimization: Efficient NOT IN execution with NULLs

* Implemented a second partial matching strategy via table scan.
  This strategy is a fallback when there is no memory for rowid merging.

* Refactored the selection and creation of partial matching strategies,
  so that the choice of strategy is encapsulated in a separate method
  choose_partial_match_strategy().

* Refactored the representation of partial match strategies so that:
  - each strategy is represented by a polymorphic class, and
  - the base class for all partial match strategies contains common
    execution code.

* Added an estimate of the memory needed for the rowid merge strategy,
  and the system variable "rowid_merge_buff_size" to control the maximum
  memory to be used by the rowid merge algorithm.

* Added two optimizer_switch system variables to control the choice of
  partial match strategy:
  "partial_match_rowid_merge", "partial_match_table_scan".

* Fixed multiple problems with deallocation of resources by the partial
  match strategies.


sql/mysql_priv.h:
  * Added two optimizer_switch system variables to control the choice of
    partial match strategy:
    "partial_match_rowid_merge", "partial_match_table_scan".
sql/mysqld.cc:
  * Added two optimizer_switch system variables to control the choice of
    partial match strategy:
    "partial_match_rowid_merge", "partial_match_table_scan".
  * Added a system variable "rowid_merge_buff_size" to control the maximum
    memory to be used by the rowid merge algorithm.
sql/set_var.cc:
  * Added a system variable "rowid_merge_buff_size" to control the maximum
    memory to be used by the rowid merge algorithm.
sql/sql_class.h:
  * Added a system variable "rowid_merge_buff_size" to control the maximum
    memory to be used by the rowid merge algorithm.
support-files/build-tags:
  Newer versions of BZR require the recursive flag in order to list all files.

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