Refactors three recursive functions

Change the code of functions include DECODE-PARAM, PARSE-PARAMS and
GET-HEADER into tail-recursive call form and add the declaration on
optimization of speed in them.
Removes the DECLARE forms in recursive functions

Removes the DECLARE forms in functions include DECODE-PARAM, PARSE-PARAMS and GET-HEADER to avoid the *unable to optimize* notes produced by SBCL when compiling. Uses one top-level PROCLAIM form instead.
Small modifications

In file maid.lisp: Add some comments before each codes which are
commented now, since they're recursive but not tail-recursive. And
reindent the codes in function SOCKET-SERVER.

in file README.md: Add s TODOs section for future plan.