Redis INFO fields

[enjoy-binbin]

By comparing the redis unstable branch code and the redis-doc code,
here are what we need to deal with (redis 7.0)

server_time_usec, added in redis/redis#8132
now in redis-doc is server_time_in_usec, added in #1455
one of them is typo, and i suppose in_usec is the right one, but...

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shutdown_in_milliseconds, added in redis/redis#9872
now in redis-doc is shutdown_in_seconds, added in #1711
typo (doc one)

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aof_rewrite_buffer_length, removed in redis/redis#9788
needed to add a comment in redis-doc

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FUNCTIONS, both missing or pending, we can handle it in #1725

used_memory_vm_eval
used_memory_scripts_eval
number_of_cached_scripts
number_of_functions
number_of_libraries
used_memory_vm_functions
used_memory_vm_total
used_memory_vm_total_human
used_memory_functions

[enjoy-binbin]

@MeirShpilraien when you have time, take a look with this one (FUNCTION INFO fields), it is hard for me to track the changes...