buffer: refactor buffer to be dynamically-sized

[cataldor]

Allow the internal buffer of libmpdclient to grow dynamically in size as needed by mpd_{async/sync}_send_command. Previously, the buffer was limited to 4KiB.

Address issue reported downstream by Debian[1] and upstream by kaliko (@mxjeff)[2] [1] https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=953110 [2] https://github.com/MusicPlayerDaemon/libmpdclient/issues/51

Hello, this is a solution to issue #51. It requires refactoring the buffer source code. I will provide another solution in case you want to stick with the hardcoded limit.