Log a warning when max connections is hit

[macneale4]

Fixes: dolthub/dolt#8942

Tested by configuring a server with 2 max conns, then connecting to it 3 times:

lcl:~/Documents/data_dir_1/db3$ dolt sql-server --config ./config.yaml
Starting server with Config HP="localhost:3306"|T="28800000"|R="false"|L="info"|S="/tmp/mysql.sock"
WARN[0000] unix socket set up failed: file already in use: /tmp/mysql.sock
INFO[0000] Server ready. Accepting connections.
WARN[0000] secure_file_priv is set to "", which is insecure.
WARN[0000] Any user with GRANT FILE privileges will be able to read any file which the sql-server process can read.
WARN[0000] Please consider restarting the server with secure_file_priv set to a safe (or non-existent) directory.
INFO[0004] NewConnection                                 DisableClientMultiStatements=false connectionID=1
INFO[0006] NewConnection                                 DisableClientMultiStatements=false connectionID=2
WARN[0009] max connections reached. Clients waiting. Increase server max connections

[zachmu]

LGTM!