Bugfix srshls memleak

[bluestn]

in function SrsHlsMuxer::update_config, the following code will cause writer whose type is IFileWriter* pointer be overrided, so need release writer before allocate a new instance for it.

if(hls_keys) {
    writer = new SrsEncFileWriter();
} else {
    writer = new SrsFileWriter();
}

[peipeiguo]

why this pr has so many files changed?