remove GET_FIRST_KMER_FROM_STORE from client

Signed-off-by: Sébastien Boisvert <sebastien.boisvert.3@ulaval.ca>
use action instead of tag for the wording

Signed-off-by: Sébastien Boisvert <sebastien.boisvert.3@ulaval.ca>
fix HTTP response to be compatible with rfc2616

Signed-off-by: Sébastien Boisvert <sebastien.boisvert.3@ulaval.ca>