Problem 1:

It was completely impossible to debug a server
action using pdb (or any other python debugger for that matter),
in order for pdb to work properly it must read the
file that it's supposed to debug from disk, since the code for server
actions is executed from memory with safe_eval, pdb can't display
any relevant information of the code being debugged.

Problem 2:

Tracebacks for safe_eval'd code were pretty useless, since all they
would do would be display the entire code that failed execution in raw
format (single-line), this is obviously very hard to debug and offers
absolutely no insight into which server action broke or where and why
did it break.

Solution 1:

Before compiling code that is supposed to be safe_eval'd, we create a
temporary file, this temporary file can have a suffix containing
information of the location of the code (e.g. '_server_action_<id>') and
where it should be stored (filestore location recommended), this file
will contain the server action code (or any code being safe_eval'd) so
that it can be debugged. This information must be passed explicitly from
the calling code to safe_eval, through a context argument which is a
dictionary with `fname` and `filestore` as keys, the former being an
indication of the calling code (e.g. '_server_action_<id>') and the
latter being the current db's filestore, so as to avoid file conflicts.
After the code is executed, the file is destroyed from the filesystem.

Solution 2:

Thanks to solution 1, the traceback will print out the corresponding
error message and the name of the file which ideally should contain
information about the calling code (e.g. 'File
~/.local/Odoo/filestore/my_db/tmphHfj53j_server_action_55') as well as
the line inside the code that triggered the traceback, effectively
indetifying where the code broke.

This commit also merges the `record` and `records` variables available
in server actions into a single variable `self`, so that server action
development is more consistent with native Odoo development, however
both record and records are still kept for backwards compatibility.

~~This commit also adds the possibility to call pdb.set_trace() from
within a server action by simply calling `pdb()`, to be reviewed later
to support multiple debuggers (py37's breakpoint() comes to mind).~~

Task ID: 54014

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