Revert "libspark/record: always fill read buffer"

This reverts commit f77184cb685c61a3f91f4c1d017b75231277b709.
Revert "libspark/record: remove debugging output accidently left in"

This reverts commit 55f742271534b6877a7d34630effd6bf2a372cc2.
Revert "libspark/record: Implement writer thread. May or may not improve anything."

This reverts commit ada3f5e24ae0df499b53f69d761b9ddfcd16aef9.
Merge remote-tracking branch 'martiis-neutrino-mp/master'