log: Make log_max_recent have an effect again.

log: Make log_max_recent have an effect again.

The log improvements in a747aeac13daf3dba43343120659e802cb569f6b
unfortunately left log_max_recent broken because m_max_recent wasn't
used anymore.

Eliminate m_max_recent and set the capacity of the m_recent ring buffer
when log_max_recent changes. In order to call set_capacity(),
ConcreteEntry needed its move constructor set noexcept.

I haven't followed the boost code all the way down but I suspect that
setting the ring buffer capacity to anything less than 1 entry will
probably cause problems, so restrict log_max_recent to >=1.

Also fix a wrong variable used for printing the max new entries during
"log dump".

Signed-off-by: Joshua Baergen <jbaergen@digitalocean.com>
(cherry picked from commit 3d59ba1671e3e76326fb706a76b8d9638d782924)