Previously, the Put() function took in a single key-value pair,
resulting in memory allocation overhead as the intermediary treapNodes
got allocated and immediately garbage collected, resulting in
performance loss due to the garbage collection overhead.
We're able to recycle the intermediary treapNodes allocated by taking in
multiple key-value pairs. This results in better performance of the
node especially during UTXO cache flushes.
Currently btcd keeps downloading blocks and fails to verify them if the
disk is out of space. We exit immediately if we detect that the disk is
out of space to ensure the database is at a recoverable state later on.
