Review and improve PostgreSQL, MySQL, MSSQL configs

[tkopets]

Review and improve PostgreSQL, MySQL, MSSQL configs

PostgreSQL

• host, port, username are now optional (can be set via environment vars and other means). See PostgreSQL docs.

MySQL

• add configurable MySQL connection charset via --default-character-set
• add --no-auto-rehash and --compress to improve MySQL startup time and improve latency on slow networks
• host, port, username are now optional (can be set via --defaults-extra-file and --login-path)
• supply password via env var MYSQL_PWD to avoid security warning
• other minor tweaks

MSSQL

• for internal commands send (and ignore) errors to stderr

Remove unused settings option unescape_quotes

Encoding-related issues #162 #164