Closes 166. Added verbose PSM messages and comments

[potto216]

ran formatter (nightly) and clippy. Clippy only gave warnings on files I had not modified. Everything is ready for the PR.

During testing ran into the below issue when testing the speed-server / speed-client. There was an Err returned and the throughput was very low 6.4 kB/s - 7.2 kB/s. I was expecting for a 1 Mbps link I would get about 380 kb/s or 48 kB/s. I'll open an issue if it is not a problem with my systems. I do not think this is related to my changes.

# Raspberry Pi comm-rpi4-01
$~/prj/bluetooth/potto216/bluer $ /home/user/prj/bluetooth/potto216/bluer/target/debug/l2cat speed-server -v 255
Listening on PSM 255
Connection from E4:5F:01:2B:04:38 with Err(Os { code: 22, kind: InvalidInput, message: "Invalid argument" }) and l2cap_conninfo { hci_handle: 64, dev_class: [0, 0, 0] } and PHYs 0b00000000000000
Send MTU is 672 bytes

# Raspberry Pi comm-rpi4-02
$~/src/bluetooth/potto216/bluer$ /home/user/src/bluetooth/bluer/target/debug/l2cat speed-client E4:5F:01:5E:2B:A6 255
Connected with Err(Os { code: 22, kind: InvalidInput, message: "Invalid argument" }) and l2cap_conninfo { hci_handle: 64, dev_class: [0, 0, 0] } and PHYs 0b00000000000000
Receive MTU is 672 bytes
6.5 kB/s   

[surban]

Closes #166