README: Add section about thread safety

Improve section comments in 'libpool-test.c'

Rename 'pool_close' to 'pool_destroy'

README: Mention alignment

Remove "Caveats" section, since now that only applies when compiling
with 'LIBPOOL_NO_ALIGNMENT' defined.

benchmark: Set SVG background to white

README: Add note about valgrind support

README: Mention pool expansion

Improve comment for 'pool_new' function

No need to use a 'struct', we can just use (or even typedef) a 'union'.