This commit adds a function that retrieves the number of records stored
in a light hash table and makes light users use it instead of accessing
the light count directly. This gives us more freedom of refactoring the
light internals without modifying the code using it.

Needed for #7192

NO_DOC=refactoring
NO_TEST=refactoring
NO_CHANGELOG=refactoring

A value stored in a light hash must be convertible to int and have such
a size that sizeof(LIGHT(record)) is a power of two. Let's remove these
limitations.

 - Use 'static inline' instead of 'inline'
 - Move GROW_INCREMENT enum out of LIGHT(record) struct

Allow to specify allocator context in matras and matras-based data
structures. After this change, the following salad data structures can
be passed a custom allocator context: light, rtree, bps_tree.

Now, names of related interfaces, global variables and
fields are based on "iterator".

This is pure refactoring without any functional changes.

Closes #1694

*) corrected light and bps_tree corresponingly

* Minor fixes of matras

* Added unit tests for matras

2) added WIN32 support to matras
3) added range allocation/deallocation support to matras

Added unit tests for light hash table