Add mtbl_reader_get_* functions to retrieve trailer metadata

[snarkmaster]

My goal was to create an in-memory representation of the original data in an MTBL file. To do this, it's very helpful to know the size of the source data, namely the key & value bytes, and the number of entries.

I added the other getters so that the "mtbl_info" program wouldn't have to use private APIs to do its job.

Test Plan:

./autogen.sh && ./configure && make
sudo make install
(for f in t/*.data; do /usr/bin/mtbl_info $f ; done) &> UBUNTU_SYS_OUTPUT 
(for f in t/*.data; do /usr/local/bin/mtbl_info $f ; done) &> NEW_OUTPUT
diff UBUNTU_SYS_OUTPUT NEW_OUTPUT 

The difference in error message seems acceptable:

1c1
< Error: lseek() failed: Invalid argument

---
> Error: mtbl_reader_init() on t/test-deb716628.data failed

[edmonds]

Hi, Alexey:

Thanks for this! Though, could you re-open this pull request against the next branch, not master? We do development on next and ff-only merge to master only for releases (so that the tip of master always matches the latest tagged release).

Also, on first glance I notice a misspelling "mtrbl" (should be "mtbl") in the doc updates. And as mentioned in issue #4 I'd like to have these getters in a separate mtbl_trailer interface, not directly in the mtbl_reader interface.

[snarkmaster]

Ok, will reopen against next