Some cosmetic changes
Fix a typo
doc: add note about latest documentation
doc: add note about GQTP and HTTP for server use

[groonga-dev,01810]

Reported by Kazuhiro Isobe. Thanks!!!
doc: add link to GQTP spec
Update mruby

4728070 ->
eeac4be
doc: add document about groonga_query_log_path directive
test: move cache_limit test to grntest tests from Cutter tests
groonga-httpd: remove groonga_cache_limit configuration
groonga-httpd: use if block instead of guard clause

Because auto database create fallback is not normal case. Top level
should have normal cases by guard clause.
groonga-httpd: indent
Support having N caches in a process

Some grn_cache_* API are exported but they are experimental.
groonga-httpd: fix cache is shared by diffferent database

Cache is used for each database.
Add missing initialization by NULL
test: remove a duplicate test

The test is done in grntest.
test: now referenced records can be deleted
doc: add grn_cache API

TODO:
  * File list should be updated.
  * It may have some syntax errores.
    I don't generate HTML from the source. Sorry. :<
doc: use :c:func: instead of :c:function:
doc: add missing c domain signatures about grn_expr
doc: suppress abuse of ".. note::" directive