Add a cache for tokens retrieved from the database

A cache of tokens retrieved from the database has been added as
a new global variable, $g_cache_token.

The chosen cache format simplifies the process of obtaining a token and
storing it in the cache, but makes it more complicated to verify
the token ID; however, in practice, this does not reduce the cache's
efficiency due to the optimal order of function calls in MantisBT.

In addition, the code explicitly casts token identifiers to integers
throughout.

Fixes 0037098, https://github.com/mantisbt/mantisbt/pull/2210