Share solver between recursive associate calls

[dparalen]

Currently each associate call in a recursive tree of calls instantiates
and loads its own instance of a solver. This is both CPU and RAM
expensive.

The fix avoids subsequent instantiation of the solver thru passing it as
an argument of the associate function.

Fixes: #3903

[ipanova]

specific example: if we happen to copy 10 package categories we'd end up to have 10 solvers in the memory instead of 1.
this pr is fixing mentioned behaviour.