Removed some redundant hint related string literals from
spd_db_conn.cc

Clean up SPIDER_PARAM_*_[CHAR]LEN[S]

Adding tests covering monitoring_kind=2. What it does is that it reads
from mysql.spider_link_mon_servers with matching db_name, table_name,
link_id, and does not do anything about that...

How monitoring_* can be useful: in the deprecated spider high
availability feature, when one remote fails, spider will try another
remote, which apparently makes use of these table parameters.

A test covering the query_cache_sync table param. Some further tests
on some spider table params.

Wrapper should be case insensitive.

Code documentation on spider priority binary tree.

Add an assertion that static_key_cardinality is always -1. All tests
pass still