box/memtx: Skip tuple memory from coredump by default

Quoting feature request

 | Tarantool is Database and Application Server in one box.
 |
 | Appserver development process contains a lot of
 | lua/luajit-ffi/lua-c-extension code.
 |
 | Coredump is very useful in case when some part of appserver crashed.
 | If the reason is input - data from database is not necessary. If the reason
 | is output - data from database is already in snap/xlog files.
 |
 | Therefore consider core dumps without data enabled by default.

For info: the strip_core feature has been introduced in
549140b3

Closes #4337

@TarantoolBot document
Title: Document box.cfg.strip_core

When Tarantool runs under a heavy load the memory allocated
for tuples may be very huge in size and to eliminate this
memory from being present in `coredump` file the `box.cfg.strip_core`
parameter should be set to `true`.

The default value is `true`.