support "gen_tcp:connect" options

[fdie]

I believe some TCP settings should be configureable :

• send_timeout : its default value "infinity" could be tricky
• keepalive
• sndbuf & recbuf : those sizes may have performance impacts

see https://erlang.org/doc/man/inet.html#setopts-2

NOTE: The SSLOpts variable in p1_mysql_conn may be renamed Opts or Options

1. "proplists:get_bool(prop, Opts)" calls could be replace by "get_option(prop, Opts, false)" calls

2. "get_option" works even if some properties are not keyvalues, for instance :

get_option(ssl_required, [keepalive, {send_timeout, 5000}, {ssl_required, true}], false).
true

[coveralls]

Coverage remained the same at 0.0% when pulling e220c8b on fdie:gen_tcp_options into 31c30c9 on processone:master.

[Neustradamus]

@prefiks: Can you look?