Spider connection string is a comma-separated parameter definitions,
where each definition is of the form "<param_title> <param_value>",
where <param_value> is quote delimited on both ends, with backslashes
acting as an escaping prefix.

Despite the simple syntax, the existing spider connection string
parser was poorly-written, complex, hard to reason and error-prone,
causing issues like the one described in MDEV-31117. For example it
treated param title the same way as param value when assigning, and
have nonsensical fields like delim_title_len and delim_title.

Thus as part of the bugfix, we clean up the spider comment connection
string parsing, including:

- Factoring out some code from the parsing function
- Simplify the struct `st_spider_param_string_parse`
- And any necessary changes caused by the above changes