Suggestion: make debug-store=database the default on Windows

Refers to 1.71.

The default [Debug Information Format](https://docs.microsoft.com/en-us/cpp/build/reference/z7-zi-zi-debug-information-format?view=vs-2019) of boost on Windows is /Z7.
From my point of view, there are quite a number of reasons to use /Zi (debug-store=database in terms of boost build) instead:

1. File size: for all the configurations of boost we have in our organization, the total file size of the boost binaries decreases from 4 GB to 2.5 GB when enabling debug-store=database.
Some examples:
For variant=debug and address-model=64:
debug-store=database: 458 MB of lib + 90 MB of pdb = 549 MB in total
default (/Z7): 706 MB

For variant=debug address-model=32:
debug-store=database: 347 MB of lib + 93 MB of pdb = 440 MB in total
default (/Z7): 597 MB

For variant=release address-model=64:
debug-store=database: 412 MB of lib + 89 MB of pdb = 501 MB in total
/Z7: 654 MB
default (no debug info): 223 MB
Here's a (admittedly ugly) overview that also shows which libraries are contributing to the binary size:

2. Familiarity: everyone uses /Zi on Windows. cmake and Visual Studio (at least since Visual Studio 2010). I first encountered /Z7 after several years of development on Windows when I asked myself why the boost debug binaries are so insanely large and where on earth are my PDBs? As /Zi is the most common option elsewhere and only few people are familiar with boost build, they should get what they except without digging to deep.

3. Deployment: There is no convenient and easily available [strip](https://linux.die.net/man/1/strip) command on Windows to get rid of the debug information embedded in the binary (at least that I know of). But with /Zi, you simply don't deploy the PDB files.

4. Technical point of view: the [pch issue](https://github.com/boostorg/build/issues/269#issuecomment-534245308) is only hidden in /Z7 because one would have to deploy the obj files according to the [Windows docs](https://docs.microsoft.com/en-us/cpp/build/reference/z7-zi-zi-debug-information-format?view=vs-2019).

5. Reliability: /Zi is not exactly maintained by boost at the moment as seen by the [issues](https://github.com/boostorg/build/issues/269#issuecomment-534245308) referred to earlier. Not many people know boost build and look through all the options it provides. So only the defaults are well-tested.

There are of course downsides I can think of (not many):
1. It might require a lot of testing.
2. One would have to fix these minor (?) [issues](https://github.com/boostorg/build/issues/269#issuecomment-534245308) (same link as before).

Any thoughts on that?